You can run clients locally today - worst case, that is always an option. The data is there and the client is really just about the user experience of interacting with it. Pretty much all the web clients can be run on your own machine, not to mention the majority of the clients don’t have to go through App Store approval as they’re web based. Web clients are interacting with relays client side (your computer is querying the relay, not the applications server) too, so they’re really not even set up to ban relays. You’re not coupled to any one client or relay - your identity is decentralized and your content is redundantly stored. What you’re describing would be fairly difficult to accomplish with nostr.