It's natural that even with open platforms, bigger instances will largely align with mainstream views. After all that's what makes them mainstream in the first place.
The big advantage with the federated model in particular is that you don't have a single authority deciding the rules for everyone. And that makes it possible for alternative views and ideas to be discussed.
As the liberal mainstream continues to discredit itself, people fall out of the echo chamber, and that's where niche communities become important.