That doesn't actually fix the issue. If Facebook is trying to set itself up like Chrome with the webplatform, or GTalk with XMPP, then they will drive the feature set of ActivityPub, whether you're federated with them or not.
Hypothetical example:
Want to see this picture/video from someone on Threads? You need Facebook's proprietary picture format, which has DRM baked in it. Even if you don't federate, Mastodon, Lemmy, etc now have to take energy away from their work to adopt the proprietary picture format. It depends on the proportion Threads takes on the network and how they can leverage that position to put pressure.
Threads currently has voice notes. Should all ActivityPub services support that? If so, do we adhere to Threads' standard or not?
This is more a question of tolerance. We know Facebook is NOT tolerant of competitors, of the open web, of free software, etc. They cannot survive as a megacorp without a level of assurance and control that they can't have if they're "just another fediserver". They WILL try to wrangle control. They WILL try to eat us all up. Why let the fox in the henhouse when you already know it's a fox?