I guess once they got big enough there was no longer a point in interoperability, it seems like a pretty big phase transition got hit about a decade ago where suddenly each walled garden got big and valuable enough that isolation was a better strategy for them.

It just sucks for the internet. I don't think, if those companies ceased to exist, that they would automatically get recreated because of some inevitable niche or market law. I think that they got where they are because of a fluke of the times they were created in -- people could not grasp how they make money.