Strictly speaking, there's no such thing as "migrating" an npub, it was never tied to any one place to begin with.

Once an event is signed, it objectively exists. In fact, your events already exist on relays all over the world, even ones you never explicitly published to. (This very reply of yours is already on a relay I run at home.) That’s what I see as the biggest difference between nostr and mastodon.

Decentralization isn’t about how many relays you use. You connected to 14 relays, but I’d guess you don’t have absolute control over any of them. I only use five, but I control two of them. So who’s more censorship-resistant?

Any service that exposes data publicly carries some legal risk, that’s a general truth, not specific to this idea. And no one’s saying every relay needs to moderate content. Public relays and curated relays can coexist.