They could offer the slickest interface and keep people locked to their friends. That interface can use protocols that make it difficult/impossible for non-Threads instances to play ball (ooh this cool new feature is only available through the Threads app; Oh, mybasement.world.ml.xyz can't read that content). There are many ways to Embrace, Extend, and Extinguish, we've seen Meta do it before (e.g. XMPP), and I'm sure we haven't even thought of some ways Threads could EEE.
I think defederation from Meta's instances is probably our only option to protect what we have.
I'll admit I checked it out just to see. Then I realized I actually don't know anybody who still uses Instagram and I didn't know what to talk about anyway, so I deleted it and blocked them from my instance.