They could well try to pull the classic "embrace, extend, extinguish" move. Appear to embrace an open standard, then start writing proprietary extensions for it, and then use their clout to get everyone to stay on their network and abandon the open source version (because everyone's on Meta's network)

in the name of growing the Fediverse.

Perhaps then we should all federate with Gab and Truth and whatever instance DeSantis ends up spinning up to organize his brownshirts?
...in the name of growing the Fediverse, of course.

If—as I believe you do—you disagree with that proposition, then perhaps you hold there are certain standards of behavior which once seen do not necessitate further waiting. I would question if that is indeed the case why you seemingly do not believe Meta has already crossed that line.

I understand your desire for "success", "acceptance". "More potential friends" on our these frothy FOSSy seas. I do. I get that.
You are not wrong to want more people to join up.

Letting in Meta is not how you do that.

If you had a house party, and you wanted it "more bumpin' " you might think the obvious answer is "More people = good so let in anyone who's willing to attend."

It is not.

Certain people—and specifically I'm thinking of Artie the Arsonist—need to be prevented from attending.

Not only should Artie and his shloshing gas can (which in fairness he has promised not to use) be turned away at the door, some of your trusted friends with guns should be stationed at either end of the street keeping Artie from coming anywhere near your party.

There should be a big giant, "ARTIES NOT ALLOWED" banner reassuring your guests (and future potential guests) that in addition to sweet tunes and chips, another reason to attend this party specifically is that they won't be burned as they have been so many times by Artie and his friends.

Meta wanting in is not a sign of FOSS success.
We do not gain anything as the "not twitter"s, "not facebook"s of the world by allowing ourselves to be co-opted back into the sinking systems we escaped.

This is an attack, and the very idea of waiting and seeing by itself is damaging to FOSS—the main selling point of which thus far has been an Artie-absent space. Even considering letting him get close enough to sniff because "...maybe he won't reek of gas" is offputting to many and damages the most valuable asset the fediverse has: having nothing to do with Those Fuckos Over There.

Assent of what FOSS is doing by the brokers/shakers it was designed in opposition to cannot change FOSS's value because THEY do not determine that.

The people who would sell your eyeballs out your skull and plant He Gets Us chips into your brain need you. You do not need them.

They don't have to modify Activity Pub directly to cause chaos. The likely order of events that everyone is worried about it the 3E's.

From wikipedia's entry:

1. Embrace: Development of software substantially compatible with a competing product, or implementing a public standard.
2. Extend: Addition and promotion of features not supported by the competing product or part of the standard, creating interoperability problems for customers who try to use the "simple" standard.
3. Extinguish: When extensions become a de facto standard because of their dominant market share, they marginalize competitors that do not or cannot support the new extensions.