Honestly, I think it would cause the same style of dustup as Meta joining. Lots of preemptive defederation, and a lot of struggle over how to integrate communities who are federating.
Me personally, I would love that. To me, the single best value prop for the Fedi is that you can basically choose your own adventure and form a lens of browsing ActivityPub for the kinds of content you’re interested in only.
So being able to, say, subscribe to /r/simpsonsshitposting on my Mastodon account would be killer. In fact it would be amazing to just interface with Reddit’s image content like I would Mastodon, instead of thread by thread.
So I guess TLDR is, I would predict that the Fedi will continue to be reactionary in nature, but it would overall be a very positive thing for the Fedi (as long as ActivityPub is not captured by corporate interests the way that XMPP was).