They could have kept the 140-char service as a free service, to raise the profile of their brand. And it probably would have been more successful without the increasingly desperate attempts to "monetise" it by stuffing your timeline with keyword-based advertising.

Meanwhile, every app or website with a messaging or notification function could have been using (and paying for) Twitter's API and infrastructure underneath. And all using a single Twitter account, so that every subscription to any of their customers' services would be a new account for them.

It could have been a great business model, but their leadership did have the imagination to try anything other than keyword advertising.