The problems you are expecting from demographic free fall aren't going to happen for generations, and at any point during that time people could decide to start having more babies.
That usually doesn't go super well [1]. Are you good with delaying your retirement 24 months for the greater good? If so, do you think that your fellow citizens would be?
Conversely, I think a lot of people would like to delay retirement and continue working, but they can't because they got let go after they were deemed to be "too old". It's a pretty common occurrence for people in their 60s. And then since your options aren't great you may as well be retired.
I'm not sure it's that easy. There are a lot of people who aren't able to work as they get older for medical reasons. And there's a lot of ageism that keeps older people out of the job market. Maybe that ageism lessens as the population ages, but we're not there yet.
The problems you are expecting from demographic free fall aren't going to happen for generations, and at any point during that time people could decide to start having more babies.