I don't think "the development" is what is claimed to be at stake here.
OP is not talking about the software, they're talking about the content. And the content model from Mastodon is not interchangeable with the one from Lemmy, Pixelfed, etc. they serve different purposes and have different models.
It's like saying that if most people use gmail for email you will switch from email to phone calls to avoid communicating with google's service. As if real time audio were the same thing as sending a message.
One thing you could argue is, instead of switching services, switching to an instance that does defederate if you dont want threads content. But that same argument can be said as well towards those wanting threads federation...
But dont think the point is what does the individual want (if that were the case, just use the option to block threads content for your user, without defederating), the point is what's best for the fediverse. I think people are afraid that something similar to what happened with "google talk" and their embrace of xmpp will repeat.
Personally, I think there's no reason to jump the gun this early... all of this post is based on a lot of weak assumptions. I dont believe that threads content would overwhelm the feeds, and if that were to happen then the software could be tweaked to prevent it.