Because there is no medium between “Hide most Very Poor avatars besides the one I show” and “SHOW THIS NO MATTER WHAT.”
I don’t think you are considering the current UX flow for those who are hiding Very Poor avatars by default and only showing them selectively. Because, to be quite honest, I absolutely despise the default fallback avatars and players are terrible at uploading their own. So I just give up and Show them.
And now I have no defense against super annoying avatars that play loud music. Sure I can hide the person, but then I’m suck guessing when they change out of the avatar before showing them again.
Hide Avatar By ID would likely only be used for nuisance avatars, and we just have to deal with it just because we have to balance between performance and seeing peoples’ avatars.