Solutions to climate change, which would also have the effect of reducing per-person emissions, need to happen within a much shorter period. By the time the population actually does shrink, having fewer people won’t make much difference
For now, it appears that the kind of robots we’d need to replace humans at scale won’t arrive soon enough to help compensate for the slowing productivity growth of an aging population
But also, we have no tolerance for couch potatoes. It is deeply ingrained into our society that these are people to be disdained, when in fact they are models of sustainable behavior. It's the people who drive back and forth to work and do other things that consume resources, especially fossil fuels, or that create the need for other people to consume fossil fuels, that we should be eyeing skeptically.
If automation were allowing more people to sit still and do nothing, then automation would be contributing to sustainable behavior (even as they might consume large resources that we should still look at critically). But if automation just forces people to scramble for ever more unnecessary jobs, possibly jobs they do not even do efficiently, jobs that are more widely scattered and need more driving, etc. ... well, then automation is making the problem worse.
Population still matters, of course. Reducing population may not fix the problem, but it's still a necessary step. This becomes uncomfortably apparent even if everyone is just sitting home doing nothing, but is worse if people are wasting resources by working. It's not the fault of the people, it's the fault of the society for having so many births. Probably some of us should not be here.
If we allow people to be born, we should commit to having them lead reasonable lives. (The present GOP position that we must force birth and then no longer care about the people born is bizarre, to say the least, and makes no coherent sense.)
How to reduce population humanely is certainly not an easy thing, just a necessary truth, almost as important as reducing fossil fuel use. After all, if the per capita use of fossil fuels goes down but population is still growing, there may still be a growing problem.
