They certainly have the choice to migrate. If they don’t want to it’s their problem. Fediverse wasn’t meant to be a wide open connect with anyone anywhere unconditionally network, if you want that go to Nostr (it’s filled with Right wing trolls and crypto/nft bros for that very reason). It’s meant to allow for instances to communicate and share content while still being run independently of one another. That also includes the ability to block other servers.