I think you're proceeding from a false, Malthusian assumption, that is that population will grow without bounds given unlimited resources.

Would you say the developed world is richer than ever ? With more food than ever ? With more stuff, more resources, more technology than ever ?

Most would.

Yet, where is the ever-expanding population growth ? It's not there. Countries like the US have to import people to show any demographic growth. Japan is facing a shrinking population because it does not do this.

The link between 'more food' and 'more people' is not, probably has never been, as clear as the Malthusian approach to analysis would suppose. So the idea that UBI is going to lead to ballooning population just doesn't seem viable, considering the resources humanity has developed in the past two centuries.

Also, the goal of post-scarcity needs to be redefined I think. Most people think it means freedom from want, but it would better proposed as freedom from need. Because as some joker always says "huk huk who gets all the beachfront property," we're clearly not going to solve that kind of scarcity. But we can make sure we live in a world where no one is starving nor do they have to work three jobs to achieve that goal, and then worry about how we'll divide up all the luxury shit later.