Would anything change on Lemmy, even if you federate with Threads?
You follow communities here on Lemmy, while on Threads you follow people. The only way for a Lemmy user to find a Threads user, would be looking for their username directly in the search bar.
In that sense, what does a Lemmy instance gain from defederating with Meta?
Yeah I believe that Lemmy has the best chances to compete with its proprietary inspiration, than every other fediverse product.
Mastodon needs endorsement of businesses/celebrities/politicians to become a viable alternative Twitter. This will probably never happen. The API changes in Twitter were far more brutal then the ones on Reddit, and many ended up joining Mastodon. Most will leave it again tho, because Mastodon does not have their favorite businesses/celebrities/politicians.
Peertube has no monetization program, which makes it even more irrelevant than Odysee.
Only Lemmy does not need huge amounts of cash or clout, to keep people on the platform. Reddit was a (relatively) democratic platform, most were anonymous and content for Reddit is easy to produce. Translating this into the fediverse with Lemmy works really well, as we can see here.
Threads will primarily influence Mastodon. Threads already eclipsed Mastodon in its first few hours. There is no need for EEE, because Threads has already decidedly won. Mastodon can not even scratch Threads. Threads is probably only interested in the fediverse, because of Tumblr. I personally think that Mastodon would gain more from Threads joining the fediverse, than Meta ever could, but we will see I guess.