Once the population declines, economic growth is not a requirement anymore.

It currently is because when the population grows while the economic activity stays the same, people get poorer and their quality of life deteriorates. When population is stable and a minority of the opulent hoards all the wealth for themselves, you need growth to keep the lower classes afloat.

Not true, unfortunately.

Economic growth is a fundamental feature of the current system that would be there even if there was no population growth.

If we improve wealth equality, slow down the economy and reduce our populations at the same time, we could very well have everyone getting richer over time.

Wealth is, when you get to the bottom of it, a thermodynamic quantity. So how exactly could we have everyone getting richer over time when there is that pesky second law of thermodynamics getting in the way?