Ask yourself, in three years from now will you be thinking "it's so nice how Meta lets me follow and interact with their enormous userbase for free, without advertising, using my own open source server and frontend"?
Remember that's the basic expectation today for a participant in the fediverse. If this feels implausible, doing anything else is very incompatible with the fediverse's existing values.
The problem isn't just that it's Meta, it's any situation where a much larger actor comes in with different motivations. Today we have a small number of users whose servers are almost exclusively run on a "community service" model. Meta is an advertising business. They are much bigger and will define the fediverse if allowed in. If we allow them to connect, it should be much later after organic growth which means we can assimilate them properly and deflect any bad behaviour.
What might happen if Meta throws their weight around? I can predict at least three outcomes
Proprietary variations to ActivityPub, probably starting with something that seems "understandable" like moderation reasons.
Certain new features get centralised on Meta's servers only (e.g. search) claiming that it's for efficiency in the distributed environment.
Claiming spam problems, require individual instance operators or their users to verify themselves with Meta to enable federation.
The question in my mind is whether their intention is to destroy the competition, or keep the fediverse alive as a way to claim that they are not a technical monopoly that needs to be broken up by regulators, in the same way that Google provides most of the funding for Firefox.
Edit: this comment changed my mind. In a nutshell, if we can't keep a large instance controlled by "the enemy" from destroying what we've got, then we just have to do better next time.
I have been making a related point that we should be concerned about any instance capturing too large a fraction of the space. I'm less concerned about the fact that it's Meta than I am about any one instance having a critical mass that gives them a controlling interest.
History has shown that those with a controlling interest eventually use that control for their own benefit.
That's why I joined a small collection of focused instances and try to subscribe to communities that are hosted in their "natural homes" instead of those on generic instances.
Edit: this comment changed my mind. In a nutshell, if we can't keep a large instance controlled by "the enemy" from destroying what we've got, then we just have to do better next time.
Yes, I would. Even if they are administered by people that have the best interests everyone at heart, sheer size means that they must be taken into account as the tools and clients evolve over time.
It's not that the system itself should be unable to cope with large instances, it's that the only reason for the system itself to gain that capability is in response to the rise or introduction of large instances. Some of what I've seen discussed is the need to change the development roadmap to accommodate the seemingly unexpected rise and possible introduction of very large instances. In other words, those instances are already controlling the direction taken.