The practical problem is resource scarcity. We're already destroying our only planet with 7 billion people living only 72 years on average. If we manage to get through the problems we've already created, it'll be a miracle, and it is safe to say that without a lot of other problems besides longevity being solved, there is zero chance that a significantly larger population would make it.

And those problems have to be solved first, we couldn't just say "well, let's multiply the population, then figure out what to do about it later".

I'm also very skeptical that, given an indefinitely extended lifespan, people would suddenly do all of those nice-sounding things you list. More likely they'd just do the exact same things they do now, for longer.

It sort of reminds me of the philosophy around the early internet: imagine if people could get access to all information and communicate with anyone, anywhere. Imagine how smart we'll become, how civilized! That's what we thought would happen, but look at what we actually did: yell at each other and watch porn. My guess is that if we lived forever, we wouldn't suddenly become interesting, enlightened people if we weren't already.