My favorite thing about SO is when a valid "solution" (workaround) stops working because the functions used get depreciated, and you're left with the original problem four years later, fucking MySQL.
EEE would require existing Mastodon users moving to threads
It would not.
Meta could implement ActivityPub in Facebook, and there's your majority population. Imagine a Facebook user "friending" your mastodon account. You follow them back because it's your mom, and so does basically everyone. Next, Facebook implements Pages to round off the concept of "communities." Then, Facebook introduces a genuinely useful extension - marketplace. Once a decent clip of people set up shop, trim off any instances that don't adopt the FriendlyPub fork.
This (along with basically all instances with communick news behind them) is a classic example of scaling up prematurely.
When this community is brimming with so much content that users start to "miss" posts about [thing x] because there are so many posts about [thing y], then you make offshoot communities, not before.
Sometimes we need a break from the war crimes.