This specific argument should never have been "defederate Facebook or not," it should have been "is defederation the right way to block Facebook out of the fediverse," but it barely even matters now. We've seen far too many admins ready to throw everyone under the bus for the sake of growth, and I don't see that stopping regardless of what happens with Facebook.
It doesn't even matter what your instance decides, you'll still wind up federating with someone who is OK with Facebook.