The reason embracing works is because it creates connections between people using the system and allows them to piggyback off of other services.
At the moment, the wider fediverse may not have a ton of people, but the quality of content blows mainstream social media out of the water. By making it available through Threads, new users are going to be encouraged to follow their normal pattern of gravitating toward the big thing while still having access to this content. If we post on servers federated with Threads, every piece of content we add is a boon for Meta for absolutely free. The fact that they have deep pockets means they already have independent federation beat on the server end in terms of stability and long-term reliability. It makes a lot of sense for the average user to just grab a Threads account and not worry too much about which other instances have the odd hiccup or potentially stop existing.
On the other hand, if people exposed to the fediverse keep hearing about all this stuff that isn't on Threads, there's a better chance that they'll get into the decentralized account model that's natural to federation. The logical conclusion quickly becomes making accounts in places that are federated with the places you want to read and post, and if Threads isn't connected to all those places it means it doesn't serve to unify fediverse accounts under a corporate banner.
Threads has a resource advantage, but we have a content advantage. If we let Threads in, the content advantage dissolves, because not only do they gain access to fediverse content, they pollute it.
Thankfully the reality is that the choice will always lie with server owners, not via consensus. As long as the owners of servers with higher-quality content and better moderation don't open the floodgates to Threads, that pocket of high quality content that a Threads account can't have will always exist.
Personally, I suspect the above will be self-perpetuating, as connecting with a larger social media entity will degrade the quality of content. The best bits will always largely be inaccessible to the big sites.