If you want to reach the ground floor in a tall building, it makes a lot of difference if reaching it by elevator, or jumping from the window. Speed matters! A _very_ slow transition probably could be managed without disruptive impacts on the individual level. But we slam the brakes in ~2 generations, such a way a large share of people alive today will be still be alive to become destitute and unsupported by lack of replacements, both on macroeconomic level, and in the micro level. If a single kid today go childless itself, he/she is very likely to become a lone senior with no close family, eventually.

A constant stream of young workers is required for a sustainable economy.

In order to pay for pensions, the government borrows money from young, working adults. This is effectively what happens in pay-as-you-go public pension systems (which is most of them, to my knowledge, apart from the US, I'm not 100% sure how pensions work in the US). The money you put in actually goes to pay for another person, with the government guaranteeing that they will do the same for you.

If the percentage of retired people increases, the percentage of working adults naturally decreases. Eventually, you'll hit a turning point where the government can no longer borrow from working adults. The government is now in a debt crisis and has to loan money from banks or foreign investors at a significantly higher interest rate, which becomes even more unsustainable if the percentage of retired people increases even more.

This is what is happening in e.g. South Korea and Japan. There are too many old people, and too few working adults. This is caused ny low birth rates over a long period of time.

What's the point of sustainable resources, stable climate and affordable real estate in a society that fades away? What difference does it make whatsoever?