"Embrace, Extend, Extinguish" is when corporate pigs try to get ahead on open standards, make their client the main one used, then make their client drop the standard.
Email is at risk since most normies use Gmail or Outlook. Google and Microsoft could agree to drop support for all other providers and people probably won't switch to the other providers.
The most Meta can do is add a load of more users to the fediverse then take them away again. For them to successfully EEE the fediverse, it would require convincing existing fediverse users to switch to threads. I cannot see that happening on here on any noticeable scale.
The worst that can happen, is a bunch of new users appear on the 'verse from threads, then disappear again.
If anything, Bluesky is the bigger threat as it touts itself as "decentralised" in order to gain users who would have otherwise gone to Mastodon, then easily pull the plug on that, and we have to wait another decade for a maniac to ruin the platform to cause people to reconsider the fediverse.
We’re up against a Lovecraftian horror. US corporate social media have the motive and the almost infinite financial means to eliminate threats to their MUA, and they’ll use legal, semi-legal, and illegal methods.
They’re also a constituent part of the US government’s intelligence-propaganda-industrial complex, which wants to track everything we do and control the narratives.
Atlantic Council » Collective Security in a Federated World (PDF)