What twitter SHOULD have done, and should probably re-consider, is leave the API open, but charge a fee to utilize it at scale. Let startups come up with creative new uses for your platform, and if they get traction - acquire them. You print money and find an easy target for growth.
Twitter's problem is their continued refusal to recognize themselves as a platform. It's probably too late at this point, but I don't think they have anything to lose.
Similar to aws? micro-pricing, or some free level? paying, say, 1c per 100 tweets for my app(s) would probably be doable. Or perhaps giving more metrics for paid accounts, or prioritized delivery?
Twitter's problem is their continued refusal to recognize themselves as a platform. It's probably too late at this point, but I don't think they have anything to lose.