40hr/week is the unsustainable part.
I think 20h/week is a more reasonable target if you want to enjoy things beside work.
Of course it's okay to work 40h/week if you really want to, or even more, if you enjoy it, but you should really think hard about whether that's really what you want, or if it's just what your employer wants you to do.
That's what I have an issue with.
When I'm working on a problem, I cannot simply forget about it at 3pm and head to the beach to enjoy life. The problem will be in my head 24/7 until one of us gives up.
But now I'm the age that he was back then, and I find I can compartmentalize or turn off work thinking easily. I'm not suggesting it's a factor of age, exactly, but perhaps some other change that occurs over time? Not really sure, but something in my head definitely changed.
This is also something you can learn through techniques that fall under what HR might call mindfulness. After a while, you have a mental git stash that you can just pop. It's not entirely lossless, but the benefits to life are immense.
It's significantly more than 50% of the time that's available to me, by the time you factor in the things that i do because of work it's getting to around 70%.
Not that many people have done it their entire lives in the way we do today. Now we have the majority of millennials with 2 people trying to do a 40 hour week along with all of the other things that need to be done. I would argue a 40 hour work week + extras is great if i'm not cooking, cleaning, doing much childcare etc.
If we look back at pre-industrial humans we'll see that the striking thing is quite how much time they spend not doing anything productive.
Terrible at math (heh) but its something like multiply 24% by 2/3. Aka 24/3=8