If you want to hear SS's Chris Best try to defend their approach, and @rabble challenging him on its implications, this is well worth a listen;
https://revolution.social/episodes/substack-ceo-chris-best-on-democratizing-media-con/
"The whole point of SubStack is this is your audience, that you can import your email list, you can export your email list, the fundamental currency is direct connection with your audience."
#ChrisBest, 2025
https://revolution.social/episodes/substack-ceo-chris-best-on-democratizing-media-con/
So ... why was Notes built as a silo, instead of using ActivityPub to connect it to the fediverse? With tools that enable you to export your social graph (lists of follows and followed-bys), so you could import it into Ghost, for example, or Mastodon, and carry on.