They don't have a diverse content-base yet. The Fediverse "shall provide it" to them if it goes according to their plans. Once this deed is done, the Fediverse is no longer valuable to them.

I'd wager Mastodon is ready for this. Abhorrent/illegal/spamming/offensive/offending instances can be defederated from to remove liability at a server-level, and users can self-moderate with "block instance" functionality from their client. Both sides are sufficiently equipped to filter their respective feeds.

It's the realm of pure fantasy to envision a world where the EU bans an original community project to force everyone into a Meta-designed fork. Mastodon users should be safe as long as they're free to run their own server and client software.

DMA applies only to gatekeeper companies, of which there's less than 20. It doesn't apply to Mastodon instances.

Further, interoperability concerns messaging apps, not social media apps.