Personally, I'm not planning on using the Meta service, but I'm not a fan of pre-emptive defederation either. The vast majority of P92 users will have 0 clue what federation/activitypub is, let alone actually log into Lemmy, Mastodon, Kbin, etc. For them, they will forever think of themselves as @username, not @username.
I'm totally fine with Meta releasing an app who's posts are exposed via ActivityPub, along with being able to consume other posts via ActivityPub. If anything, I would like to think it'll drive more people off the Meta platform and into Mastodon, as moving to a federated app doesn't mean they have to completely break connections with their network on-platform with Meta.
Overall, I'm more in favour of allowing a personal user to choose to defederate from specific instances, because regardless of what happens, if Meta joins, there will be other companies getting on the bandwagon, and endlessly splitting up based of which instances federate with which others will eventually lead to the whole damn thing falling apart and the big players becoming the de-jure instances anyways.
I mean, the vast majority of Lemmy/Kbin users migrated from Reddit, as did the vast majority of Mastodon users from Twitter. I'm fine with keeping things open to help facilitate more user growth to community run instances, while also having a place for the less tech-savvy to get their feet wet.