Good point, young people just out of college who have lived off of their parents and the government for a quarter of a century might get upset about having to support non-productive people.
People will finally figure out that they're only productive for about a half their lives, and that it's not just the last quarter that they need help with, it's also the first.
The real problem is the transition period where there are more unproductive people than not, because the stock of existing people is still here. That's why a crashing population is bad.
The question is if the complex society we have built actually requires growth to maintain it. In other words, our society requires 0.6 lives of work rather than 0.5.
If this is the case, even a steady state would mean a lowered standard of living.
It's hard to know, as our societies have decided to reduce costs on the front end (lowered birth rate) rather than on the back end (decreased longevity). That's understandable as someone who is never born doesn't suffer, but it's the worse solution from a social maintenance standpoint.
The real problem is the transition period where there are more unproductive people than not, because the stock of existing people is still here. That's why a crashing population is bad.