The answer to this comes in multiple parts and some would count as spoilers so I will only talk about the part that isn't, which is the effect that finding thousands of new planets has had on Belters. The series has touched on this already, but only very lightly (and last season, rather than this season so far).
We see the effect on Mars (exodus that is starting to threaten the entire Martian project) and on Earth (exodus, but in a way that is relieving Earth of population burden and opening new economic and political opportunities). Belters are reacting differently, because they have a different problem: most of them cannot go to most of those planets, ever, due to the gravity that would damage their bodies, but even if that problem is overcome, even if Belters settle some worlds, that just means they become planet-dwellers, and not Belters.
Inaros sees this. He sees a future where humanity is stretched over thousands of planets and Belters are a forgotten remnant of a people clinging to some irrelevant rocks in a single star system - a galaxy of Inners.
His planned solution to this (or at least a part of it) is to make people afraid of living on planets.