You just wait. The closed services will close down or become hostile enough that people will migrate. Not everyone will, but over a longer period - enough.
People getting into Solid and ATproto today are like people using own XMPP servers decades ago, or Mastodon years ago, or Matrix. Some projects like that will succeed, others will fade. But one day, you won't be able to post to Discord due to some policy changes and you'll have to reevaluate options.
Also, you can't backup from Twitter anymore. Or Discord. Or google photos. Or many others - they cut off that option once they're big enough.
> You just wait. The closed services will close down or become hostile enough that people will migrate.
I've been waiting a long time. Over that time, the closed services have only gotten more popular and no regular person is ever complaining that they are "hostile".
Regular people don't like ads, but they dislike paying even more, so they're pretty OK with the status quo. They certainly don't want to be paying for a domain name and paying for hosting.