He's winning a power struggle that shouldn't be taking place.
First, Costolo pushes out Twitter's COO. Shortly thereafter the head of media Chloe Sladden departs. This makes it appear as if C-level personnel is the problem and if Costolo can assemble the right team, under his leadership, Twitter can win. But this isn't so.
Twitter has core problems nobody can fix, described HERE, HERE and HERE. A series of snobby cliques that discourage engagement. An, as TheStreet's Herb Greenberg calls it, "arcane" platform. And advertising that amounts to worthless spam any client with a clue should be able to reproduce for free.
Anything good (or bad) that happens on Twitter happens organically because of Twitter's users, not its employees. Delusional and egomaniacal Twitter executives didn't invent, say, second screening during television programs as much as they like to act like they did.
Memo to Chloe: Get over yourself.
But, clearly, Costolo's ego doesn't see it this way. As such, it sure as heck won't let him do the right thing for investors and Twitter users and ...
The stories at the links -- and the links inside the stories at the links -- chronicle one part of the narrative I've been assembling since 2012. As Twitter goes up in smoke, hopefully somebody will finally listen.