This is where federation matters. I can choose where I host a Matrix room or Mastodon account and other people using other service providers can interact. It's even possible to move a Mastodon account from one provider to another without losing contacts, though this looks to have some rough edges.

I don't think everyone should necessarily own their platform, but it's very valuable to have the option to migrate anything that's a critical part of your identity or business to a different provider.