In my opinion, "watch and see" is a good approach towards potentially hostile entities. Meta is not "potentially" hostile - it is a hostile entity already, due to its backstory of EEE (embrace, extend, extinguish), and it should be told to fuck off right off the bat. It will not contribute with the Fediverse in the long run.

Their strategy is potentially along those lines:

1. Create its own instance (let's call it "Metadon"). Play by the rules at the start. Get a rather large userbase.
2. Introduce policy, code, and monetary changes to Metadon; things that don't piss off the users, but make it harder for admins of other instances, that'll need to either play along or defederate Metadon. Over time smaller instances will bleed to death, with Metadon absorbing their userbases.
3. Once Metadon controls a good chunk of the Fediverse, it pulls off the plug, because it's now in a position to piss off the userbase to further Meta's goals. "We're going to defederate everyone else for protection of the users. Think on the children!" Then it starts reintegrating into Facebook and WhatsApp, including crap like "you need a Faecesbook account to use Metadon".

And we can't really rely on "everyone doing the right thing", because most people won't. Cue to the users still using Reddit because they don't care about things in the long run; it would be the same here, plenty users will use Metadon if it throws them a big enough of a bone.