Yeah I (and probably everyone else) agree that indefinite growth is not sustainable, but no-one argues for such growth and as far as I know there are no reasons to suspect the world population will grow indefinitely.

I don't know of the top of my head how sustainable a lower-middle class Westerner is, but my guess is not overly sustainable, as it feels that modern society is made so you naturally emit quite a lot. My guess is that we could sustain 10 billion or a bit more, I haven't really heard any convincing arguments we couldn't. I agree there must be an upper limit, but I think it much be much further than you think.