Plumbers pick their own work conditions as they are often self employed. So uh kinda? Mostly not. Heavy things are still heavy and still need to be lifted. You're probably not going to bring in a jack.
My best friend was a HVAC repairman and is a programmer now. He's happier as a programmer. Some people are happier as an HVAC repairman, and more power to them. However it's not the norm for people who have done both.
Plumbing isn't the only kind of blue collar work, but I expect even there the work safety has improved considerably. Many things that used to be heavy are now light (copper and lead vs PVC and other plastics), many things that used to be toxic are no longer required or have safer alternatives (e.g., solders), and in general the cultural expectation for a plumber to risk life or limb for a job is far lower.
And if you're working in a factory, conditions have improved far more drastically. Worker safety is serious business given OSHA and legal liability associated with workplace injury.
Happy to hear about your best friend; I doubt it extrapolates, but it doesn't matter much to me--I don't have a dog in the fight.
My best friend was a HVAC repairman and is a programmer now. He's happier as a programmer. Some people are happier as an HVAC repairman, and more power to them. However it's not the norm for people who have done both.