I think the issue is that on most people's feeds, the vast, vast majority of the content that they see would be from the @threads "instance." Think of how salty people get about the size of mastodon.social or lemmy.world are compared to other instances, and multiply that along with the threat of a poison pill in the form of corporate embrasure.

Culturally, the fedi is pretty anti-corporate, so a lot of members are suspicious of centralization / partnership with corporate entities. Though this lens, I think the objections make total sense.