Most of the YouTube channels I follow have no external means to notify me about new videos. If I were to stop using YouTube, they were banned, or YouTube subscriptions stopped working the same, they would have no way to actually contact me about new content, and they’d lose me.
Effectively YouTube owns the relationship between the creators and I. This can be changed, CGPGrey has been building a mailing list for this reason, but it takes a lot of painstaking deliberate action to achieve. Migrating to a new platform alone is insufficient.