People are complaining that facebook is, intentionally or otherwise, going to suffocate the baby in the crib, before it has a chance to grow. If you provide a way to see mastadon content without actually doing the work of joining mastadon, nobody will join mastadon except the people who have strong ideological reasons to join mastadon, which accelerates the problem.
It's the same effect as any platform that tries "free speech" invariably becomes a nazi echo chamber, because the only people who WANT to use the less popular system are those that CAN'T or REALLY REALLY REALLY cling to their ideology.
The original idea of having many small-sized instances is already failing, also without Meta. Small instances are unreliable (they quit/shutdown), have major sync issues (not seeing all replies, boosts etc) and have a tendency for too restrictive moderation and defederation.
So indeed, most people (normies) will naturally flow towards larger and more mainstream instances. It's already tilting in that direction and actively encouraged in the signup process.
As these instances grow, they will simply have more disagreeable posts (from the perspective of the ideological instances) leading to even more defederation, hence the split will become ever harder over time.
It's the same effect as any platform that tries "free speech" invariably becomes a nazi echo chamber, because the only people who WANT to use the less popular system are those that CAN'T or REALLY REALLY REALLY cling to their ideology.