> This is probably more than most people are willing to accept
You say that, but this a majority opinion amongst people I know both in my own age group (late twenties) and my parents age group (late fifties). My circles probably skew liberal, but even so I think that most people now have first-hand experience of elderly loved ones with dementia and the effects of that on both the person themselves and those who have to care for them. I can't see many people wanting that either for themselves of their descendants.
> I don't think we can sustain a society where people work for 40 years and then are retired for another 40. Quality of life will need to drop substantially, or we'll need massive birth rates.
And massive birth rates aren't sustainable either. That only pushes the problem back a few decades. This is why I'd ultimately prefer to my life to be cut short. The only alternative on a societal level is to work much harder while I am alive. I'd rather enjoy my time here and then go and let others have their turn.