The economy steadily shrinking because of a graying population is going to put environmentalism on the back-burner indefinitely, making our environmental problems much, much worse.
It won't keep graying indefinitely, that's mathematically impossible.
But many 1st world populations continue to increase, despite (for some time now) sub-replacement fertility.
Immigration.
This planet makes no sense without humans.
It makes perfect sense with less humans though. There wouldn't be anything wrong with settling on a billion people.
Why does it need to make sense
Because life is progress. The universe would make no sense without anyone utilising it from gametheoretical accounts.