Their goal is be compatible with the old fediverse, gain all the users by showing all the instances content, then become incompatible with the old fediverse and kill it. Then at that point there is no more fediverse because Threads by itself is not a federated system.

I firmly believe this is the case. They will lure in users with things like video hosting and better spam filtering, while making it increasingly harder for the fediverse to exist (for example by participating in the development of the common interfaces and making them increasingly complex), then eventually just closing off altogether when they have large enough user base.

Having corporations participate to fediverse will only be a bad thing, apart from some short term gains.