The problem is rates never stay the same. What you say is correct, if nothing changes. But things do change (for one, population is decreasing).

Let me put it another way, if a pandemic would reduce the world population by 50%, what do you think birth rates would do?

Now, if your argument is "it will soon be a shitty time to be old and it will last for a while", with that I do fully agree. The old people of today have it much easier than the old people in 30 years. But people in 80 years might have enough (resources, space, etc.) to start making children again at different rates.

I would say we will live and see, but doubt it is the case for that long periods...