Why joke, merchandising is good business. Look at the Star Wars franchise, over 30 years it's earned an estimated $22B of which $6.68B was generated at the box office. The largest chunk comes from merchandise ($9B). [1]
Granted, kiddies might be less interested in a plush fail whale than a plastic light saber.
Why, the Twitter service itself. Its called lock-in.
Rewrite the licensing to say for non-commercial use only, except under special license. Allow businesses to opt into a licensing agreement for a few hundred bucks a year. Release the hounds on any unlicensed companies.
"What possibly" and "conceivably" assumes that the current form of Twitter is all it could ever be.
If they're smart enough to do Twitter, they're smart enough to enhance Twitter in a way that people will pay for it.
Keep in mind, this is all based on the linked article's characterization of Twitter as an ad company, and what they might do instead.