After a certain point, practicality takes a back seat and other concerns become primary.

I love my Mazda 3. It does everything I want it to do. Looks great, drives fast, has a loud stereo. But I will eventually replace it with a soundproof car. Why? Because I plan to move in town, and my car has already become the one place I can go to listen to music loud on demand without having to resort to earphones. Once I move in town, I won't be able to just hop in my vehicle and crank it up. I have to get out of the parking deck, get out onto the highway if I don't want to be a douche about it. It's not a refuge anymore.

I can't not move in-town, and I can't not have my refuge, I rely on it. Without it my stress level goes up, and when you have a lot of money, the best use of it is to reduce stress. A soundproof car is not a 'need at all costs' sort of luxury that a private plane becomes for high-profile CEOs that have a serious need to not deal with commercial air travel, I can get by without it for awhile, but at some point the advantages will outweigh the tradeoffs and I'll spend a gross amount of money and dump a perfectly good automobile for one that solves a very specific need.

Well I guess the crux is the difference between what one considers 'major differences'. It's true that a 25k and a 75k car will bring one or multiple persons from point A to point B. A $500 clunker will do that, too. 25k will get you (sticking to new cars for the sake of the discussion, but the same dynamics hold for used) a Camaro, a Dodge Grand Caravan or a Ford Escape. 75k gets you a BMW X5 or a Porche Cayenne Hybrid. The room in those cars, the level of comfort, the quality of the drive is of a different level than in the first 3. Is it 'worth' it? Of course that depends on your frame of reference. For somebody with a commute of an hour, or somebody who regularly goes out to ski or on longer road trips to visit family, it's a real difference.

Back when I was a student, or even when I just had my first job, I looked at cars in that price range and thought that people would have to be nuts to spend that much on a car. Now that I'm in my mid 30's with a family, the BMW X3 I drive doesn't seem like such an extravagance it once did. If I had 2 million I too would semi-retire, but the X5 or Cayenne would be out of my price range - the yield on 2 million doesn't make it fiscally responsive to buy a car of that price range. But somebody who retires with 10 million could. And they could dine in much nicer restaurants much more often (for those into that), and do a bunch of other things that would be a real difference from those with 2 million.

Which brings me to the second point - what's the difference between having 1 million or 10? Well quite a lot. 1 million will let you 'retire' if you're content with a lower-middle class lifestyle and do some paid work you enjoy every now and then. 10 million lets you retire comfortably and lets you go on holidays abroad a few weeks or months a year. It still 'only' (yes yes, first world problems) affords you a house in a nice but not upper class neighborhood, and you won't be jetsetting first class around the world with it. Is that 'necessary'? Of course not, billions of people would literally give their right leg for just a fraction of these amounts. Yet most people in my (Western European, mid career professionals) social circles don't consider it 'wealthy' or would be content with such a lifestyle for the rest of their lives, without any prospect of advancement.

Maybe there is a similar case for the difference between 10 and 100 mil, or 100 and a billion, I don't know - I don't have that much money, most likely never will, and I don't hang around with anyone like that either. But I can imagine that the people with the 50 feet boats in the marina (the 100 million crowd) have a different lifestyle from those with boats that can go from Miami to Monaco (the 1 billion crowd).

I don’t agree that you need to stick with new cars for apples to apples comparison; it’s just a question of value for money.

I agree there is a large bump from a 500$ Junker that's likely to break down and say a 25k used 2013 Acura TL with 15k miles. However, the gap from a 25k 3 year old used car in great condition and a 35k new car of the same model is not that great. And the gap between a 35k car and a 70k or 250k car is again there but not that huge. The 25k is long past the point of not breaking down on the side of the road; it's got heated power seats, heated power side mirrors, auto dimming rear view mirror etc.

Don't get me wrong the next bump is real and say automated cruse control is useful. But, it's less useful than you might think and you can expect that to be part of the basic 25k package before too long.

As to retirement I am saying early retirement on ~5 million where driving a 25k car is a completly reasonable choice is where dimmining returns start to hit hard.