Considering that Parler was curbstomped by Google/Apple/AWS last week, it's safe to say that Twitter doesn't operate in a naturally competitive market.
Parlor was incredibly lazy and opportunistic. The massive data leak and slamming by one of the most notorious sites on the internet, The Pirate Bay, suggests strongly that Parlor was not pursued in the most competent manor. [1] [2]
There are dozens of webhosts other than Google/Apple/AWS and plenty of ways to collocate directly. Even if you wanted to host something no provider could touch, you could publish a front
Picking some of the most recognizable and PR influenced companies in the world to host your publicly and deliberately controversial content is just plain stupid. They could have just seen how the pirate bay responded and copied them. Heck, since Parlor didn't host anything actually illegal, they could have potentially used the .swiss or .ch TLD since Switzerland has pretty strong anti-censorship laws.
Considering that Parler was curbstomped by Google/Apple/AWS last week, it's safe to say that Twitter doesn't operate in a naturally competitive market.