I agree it would be a dangerous precedent.
Thing is, though, every instance is not equally valid and legitimate: that's the reason for defederating from Threads.
Not sure what you mean by what Gmail and Microsoft did to email? Do you mean that they assume many unknown email origins are spam? Though Gmail's obviously attracted a lot of users, and I myself have moved off it now to paying for my email provider elsewhere, I was under the impression it's been quite good for email and for pushing secure email, and being good at anti-spam.
So, I didn't mean instances treated unequally in the grand, set-in-protocol scheme of the fediverse - as if some centralised authority/agreement that this instance counts for more than that. Just as defederation doesn't make meta's instance authoritatively illigitimate.
But an instance can choose, within that instance, to defederate with another; likewise an instance within itself could deprioritise some or all others' instances' votes.
Still agree dangerous precedent ...but still wonder if some sort of instance-controlled moderation of external content is eventually necessary in the future. Or, I suppose, there could be separate services (much like ad-block lists) that users individually could enable to auto-moderate/adjust their own feeds.
And (sorry for waffling!) I suppose it depends a lot on how much you browse specific communities and how much you scroll "all" or whatever. Back in the before-days, I'm used to subbing to very few communities, and generally lazily browsing r/all