There's nothing preventing users from moving to another DM client. You can always go back to the old client or export or copy paste message histories if you need them.

It's not comparable to Twitter or Facebook, because you own your identity key and can always change the client or relays.

There's always a tradeoff between availability and security of messages, and maybe there are different use cases for both kinds of messaging. On Nostr, we are free to choose.