Why not judge these instances on their own merit though? If what you say becomes true and is so problematic and rampant that it needs addressing, you can block that instance. But doing so preemptively seems petty and counterproductive at best.

What if there is an instance that selectively reposts from Threads only decent, thoughtful discussions?

Oh and as a side note; if you’re worried about stuff getting more mainstream, toxic and polarized that’s kinda inevitable if you want more people using the fediverse, that’s just how it is when lots of differently thinking people are in one place.