Reasons you might want to address page numbers: Jumping ahead helps with skimming. You might (roughly) remember where you left off last. Other threads and page numbers are referenced in other threads (yes, people could link, but they don't always do)

I also like page sizes as "breaks" to follow up on links etc.

Discourse's implementation of infinite scroll isn't all that bad (it keeps position properly and doesn't seem to have strange glitches, although I haven't tried it on a bad connection), and at least navigation elements are there. But they feel strange and trip me up every time: the secondary "scroll bar", or in thinner browser windows the "progress bar" at the bottom that overlays the post content and you have to click to get the scroll bar pop up. It feels like a mobile app, half-way designed for touch, not a forum.