You've had the US President tweeting a lot. I'm sure the US government could replace Twitter with a bespoke version of Twitter if they didn't want to use Mastodon. Similarly there are news publications like the NYT which could operate their own bespoke Twitter. Reuters and the Associated Press basically already manage some services which could be described as a bespoke Mastodon instance. Who are the Twitter users who matter who can't afford to operate their own Mastodon server?
I'm not suggesting any individual do so, but this is not rocket science. I feel like maintaining a physical building is probably harder, so any organization with a physical building that has ~100+ members should be able to do this - and enough organizations doing this could replace Twitter.
But this will never describe a significant portion of Twitter users. It might work for some, but it wont replace Twitter.