that's an interesting question, I think spam is part of how... for a proper answer I would look at how SMTP has evolved over time.

Consider how worried people are about Google Chrome's browser market share, and how Google is increasingly able to dictate details of the web unilaterally. The Web is made of standardized protocols, but Google has significant control over it.

Consider the state of email, a standardized protocol, where Gmail is dominant, and anything that can't deliver to Gmail is practically irrelevant, no matter whether it follows the protocol.

Now consider Bluesky. If Twitter supports the AT protocol early, it would be by far the largest implementation, meaning that Twitter practically controls it. It would be like Gmail for email, except there are no other options to begin with. If Twitter does not support AT protocol early, then AT protocol will be irrelevant, and other networks like the ActivityPub Fediverse will continue to outpace it.

By being the gatekeeper of its evolution of course.