Inevitable unless its not. Providing the infrastructure for say, facebook's primary needs is relatively cheap, but nobody can compete due to the network effect + them buying up any large competitor.
Its because everyone is already there that this stuff is allowed to fester. Nobody who's not crazy right wing is going on Parler - that is the way it should be. It should be totally self-selected which grouping you use, and there shouldn't be one thing that has "everyone" that prevents all others from starting. White supremacist websites existed well before this stuff, but in a decade they didn't get the traffic that one popular FB post gets.
Where's "Apolotical" Facebook? Where's "only share photos of your kids and vacation" Facebook? They can sell ads for baby clothes and cheap flights. There's more of a market need for that sort of thing than what it is that Facebook provides.
Here's a solution - a bunch of competing "profile" companies that you can choose to manage your online profiles that would work on any social network. They get kickbacks from those social networks running ads. Some of those profile companies might be incredibly private, some might give out a ton of info about you for targeted advertising. Regulate it so companies must have X and Y user access controls on your user profile, and also so sites must use those sites.
I don't even think the above is actually a workable solution. But the idea that things are inevitable is a lack of imagination.