My problem with Lemmy is the lack of activity in niche communities. You’re right that there needs to be a critical mass and arguably Lemmy has it, but only for the most mainstream, generic type of content. It doesn’t have the mass to sustain any sort of niche, outside of maybe tech related topics because of the way the userbase is slanted.
I find myself going back there often because of that, but I hope that the userbase for generic content enough to sustain and grow, from where more active niche communities can spring up.
The most impactful single decision one can make in terms of not making things even worse is not reproducing. It is the single most sustainable decision in terms of resource use, pollution and climate change.
The fact that people are voluntarily doing this is good, the presupposed “dangers” of population decline are dangers to the systems that demand infinite endless growth, which I would argue are dangers to people in the first place. Fuck 'em.