> as an open protocol, they literally cannot do that.
From a cursory search, it seems the AT Protocol is solely maintained by Bluesky. Also, AFAICT, Bluesky operates the only sizable/relevant servers. i.e. Neither the governance nor the technical operation are decentralized. (edit: Although the OP call for projects includes options which would probably lead to further decentralization, that doesn't speak to the present state.)
Note that I find it reasonably credible that Bluesky won't do that. I just don't see why they cannot. If there is a massive incentive to make backwards-incompatible protocol changes, say a fundamental security flaw, wouldn't the expected outcome be that Bluesky unilaterally makes those changes?