Over the last year I've roughly doubled my pushup strength, with very visible resulting muscular hypertrophy in my triceps, back, and shoulders (even though I was optimizing for strength rather than hypertrophy). The total time taken over that year has been about 6 hours: three 40-second sets of pushups every two days, with 2⅓ minute breaks between sets, which I'm not counting because I can post to HN and drink yerba mate during that time. This works out to one minute per day.
There is literally nobody in the world who has less than 6 hours of free time per year. This is not a matter of economics; no economic system is so all-pervading as to sell every single minute of your day.
I started out doing three sets of knee pushups to failure, and once I reached ten reps I switched to real pushups; once I reach failure on the real pushups (3–5 reps in the first set, sometimes as little as 2 in the last) I continue with knee pushups until failure. This takes about 40 seconds and seems to be a good balance of intensity and safety. The only equipment it requires is a reasonably clean floor or patch of grass, so you don't have to buy equipment, pay a gym membership, or even walk to a different part of the house. You can do it on the train while commuting to work, at the bus stop while awaiting the bus, in the break room at the office, outside your car in the parking lot, in the park when you walk the dog, or in your bedroom after getting up, unless your hoarding problem is even worse than mine.
I'm slightly obese (109kg) like most of the population in rich countries, and I think my state of muscular development was about average in that context. Calisthenics permits increasing resistance to almost arbitrarily high levels, so you can keep the intensity high and the workouts short even as you get stronger. Stronger people would presumably need to invest more time than one minute per day to make further progress, perhaps as much as ten minutes or even more, but those aren't the people we're talking about.
So what's missing? It might be inspiration, discipline, executive function, hope, knowledge, wisdom, or some combination of these. But it's not time or money.
I fully agree that these simple exercises can get you quite far. I have made similarly good experiences by aiming for 50 cleanly executed push-ups per day. But you will soon have to diversify them, though that requires simple equipment only: a high horizontal bar for pull-ups and dead hangs is surprisingly hard to find outside of a gym.
Aerobic exercise, which should really also be part of a workout routine, is significantly more time-consuming though, and requires proper equipment. Most importantly a new pair of good running shoes every year or so to reduce wear on the joints.
So yes, even with kids or similarly demanding circumstances it should be possible to accommodate moderate exercise in most peoples lives. But the result probably won't be body-builder levels of muscular development.
I pretty much agree, though I do have a few thoughts to add.
One is that if you're doing 50 pushups you're probably doing endurance training rather than strength training, unless you're talking about doing 10 sets in a day. And if you're doing them every day you're going to build strength very slowly or actually decline in strength. You need recovery time to build muscle. If that's what you're after, do however many pushups every other day. You can work on your legs in between if you want. My legs are still pretty decent from when I used to commute by bicycle in San Francisco.
Another thought is that diversifying from extensor exercises into flexor exercises isn't even as hard as you make it out to be. It doesn't even require simple equipment.
You can bicep-curl the groceries.
You can do a pullup on a doorframe, if you can find a doorframe that won't break.
You can do one-arm inclined rows with a clothesline pole in your crotch. If you don't have clotheslines in your country, use a telephone pole.
Guys in prison deadlift the bed, or each other.
Kids climb trees and hang from horizontal tree branches. You can do that too.
If you lie down on your back under the kitchen table, you can grab opposite sides of the table with your two hands and lift yourself up with your biceps that way. If this isn't enough resistance, you can do archer rows that way. Putting both hands on the same side of the table may destabilize it, depending on the table.
I have a metal-framed transom over my kitchen door that's tall enough that I can dead-hang from it.
Fences and walls are commonly high enough that you can dead-hang from your hands on them too, though they may not be ideal for a pullup.
If you have a door, even a hollow-core wooden door, you can open the door, support its distal side with a wedge of wood (or newspaper) to take the load off the hinges, and then you can safely hang from the top of the door. This will not work with an aluminum screen door or a car door, but otherwise you're good.
Unless you live in the Gobi, you can tie a rope around a telephone pole or a tree, then climb the rope.
You can pull up to a ladder rung from underneath the ladder.
Playgrounds have jungle gyms.
Finding objects strong enough to hang from is a little harder than finding a floor, but still not a category that requires exercise equipment specifically built for it.
As for aerobics, for me the best aerobic exercise is dance, because running is boring and I don't care for the social dynamics of team sports, and running shoes are generally not helpful for dance. Many forms of dance are done barefoot; others usually use flat-soled shoes with no cushioning, on purpose, because cushioning dramatically impairs your balance. (I agree that cushioning is very important not just for running on concrete but even for extended walking on it.)
(Actually, swimming is even better, but I live too far from the river.)
Many, perhaps most, people in rich countries are experiencing levels of physical disability due to muscular atrophy that could be corrected by exercise averaging on the order of one minute per day. It's true that, to get body-builder levels of muscular development, you have to treat it as not just a full-time job but also a weird cult that fanatically controls your diet. In between the literally crippling levels of sedentarism so many people suffer, and eating kilograms of meat three times a day except when you're cutting, there is an enormous spectrum.
I'm aware of the need for regeneration. Fortunately (?) I am not actually disciplined enough yet to do it every day; more like three days in a row and then a rest day. And other exercises in between sets to make use of that rest time. I'm not gonna aim for higher reps, but will eventually elevate my feet to make the push-ups harder.
Supporting the door to protect the hinges is an excellent suggestion, since concern about the hinges is exactly why I originally hesitated from using a door! It seems creativity is indeed the true limiting factor in choosing equipment and adapting exercises.
I think I will pick up running (many acquaintances are in a runner group), but dancing is also on my to-do list.
Over the last year I've roughly doubled my pushup strength, with very visible resulting muscular hypertrophy in my triceps, back, and shoulders (even though I was optimizing for strength rather than hypertrophy). The total time taken over that year has been about 6 hours: three 40-second sets of pushups every two days, with 2⅓ minute breaks between sets, which I'm not counting because I can post to HN and drink yerba mate during that time. This works out to one minute per day.
There is literally nobody in the world who has less than 6 hours of free time per year. This is not a matter of economics; no economic system is so all-pervading as to sell every single minute of your day.
I started out doing three sets of knee pushups to failure, and once I reached ten reps I switched to real pushups; once I reach failure on the real pushups (3–5 reps in the first set, sometimes as little as 2 in the last) I continue with knee pushups until failure. This takes about 40 seconds and seems to be a good balance of intensity and safety. The only equipment it requires is a reasonably clean floor or patch of grass, so you don't have to buy equipment, pay a gym membership, or even walk to a different part of the house. You can do it on the train while commuting to work, at the bus stop while awaiting the bus, in the break room at the office, outside your car in the parking lot, in the park when you walk the dog, or in your bedroom after getting up, unless your hoarding problem is even worse than mine.
I'm slightly obese (109kg) like most of the population in rich countries, and I think my state of muscular development was about average in that context. Calisthenics permits increasing resistance to almost arbitrarily high levels, so you can keep the intensity high and the workouts short even as you get stronger. Stronger people would presumably need to invest more time than one minute per day to make further progress, perhaps as much as ten minutes or even more, but those aren't the people we're talking about.
So what's missing? It might be inspiration, discipline, executive function, hope, knowledge, wisdom, or some combination of these. But it's not time or money.