if it becomes really big, then every federated instance would find itself coping with large amounts of traffic passed to and from the big instances, and it will become difficult to run a small operation cheaply

I think that' where the biggest threat lies. How is a small operator going to keep up with the demands of a corporate server cluster with millions of users. A small operator would have to defederate. That puts us back to the crux of the original question, should corpos be allowed on the Fediverse. Why not save everyone the circle jerk and blacklist them from the start.

A secondary threat is corporate sabotage of the ActivityPub protocol. They already have a track record of doing that to free and open standards.