If not, can they shut down and then only come back for users using non-grandfathered terms?
If so, can they shut down for only users using grandfathered terms?
Aren't we now back to where we started?
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I disagree pretty fundamentally that you have any right to have a company continue to host content for you (which is equivalent to having a right to having a company repeat what you said to anyone who asks - which is clearly a free speech violation) short of them signing a contract with you guaranteeing that.
I agree that as a user it's very frustrating when something you use changes what it is, but I don't see how a social media platform is any different from a bar in that regards. They both derive a lot of their value from their users, it's frustrating for the users of both if they fundamentally change who they are, but it's their right do that anyways.