What solution is there, if not monopolies or decentralization?

One solution would be smaller cathedrals. Not everyone wants to go to the bazaar to pick what works for them; but most everyone would benefit if there was a limit to how encompassing a tech company can get.

Network effects are also somehow not working very well for decentralised things, possibly because everyone wants to fork Product X but no one wants to improve it for added discoverability or less bugs.

there are different solutions to different problems.

for example Substack is a good upgrade to social media. It's centralized, but people have to pay their favorite writers. They do this to avoid bs, ads, being a product. The important part of Substack is that if the platform screw the authors over, they have control over their email lists (unlike social media), and they can just move somewhere else.

So a platform like Substack is a move in the right direction vs. something like Twitter.