Substack's incentives are even worse than YouTube or Facebook.
If you think YT and FB were good for western democracy, then I guess you can die on that hill.
Otherwise, die on the hill of "fuck those profiteers and the very specific capitalists very much among us right now in this exact setting that enable them, who should probably know better by now"
There is a difference between fighting censorship and supporting a platform of the day. So fight censorship, don't support substack, YT, FB or anything else.
On the side note: having many competing platforms help to fight censorship. So if you are serious about that I don't see how you can't support and promote diversity. Any road will start with the first steps anyway and having 5,10,50 platforms is better than having 3 platforms and seems like a good first step.