I think you're really underestimating the value that "normal people" provide to Twitter, I would bet that the majority of Twitter users (that are active) are not "early adopters", but "normal people" that follow celebrities and talk to their friends. I left high school a few years back and the majority of my Facebook friends are ~19-20, almost all of them use Twitter and not a single one could write a line of code in any programming language, nor could they give a damn about whether or not Twitter is open to developers.

Take a look at the Twitter trends list if you want proof that Twitter caters more to "normal people" than it does "technology" people, here's the #1 trend as I type this: http://twitter.com/#!/search/%23NameATurnOn

Yes, their only defense is their network (which is, in my opinion, their "product"), but the majority of their network is not going to leave because of their developer relations. Twitter are safe as long as an alternative doesn't exist and they provide value to their users.

Twitter aren't stupid, they're not going to be enacting this plan if the majority of their users use third party clients. I suspect the majority of people that do use third party clients are those using "value adding" third party clients, like Hootsuite, and those that use Hootsuite are the people that get value from Twitter's network (for example marketing people) so if Hootsuite shuts down they're not going to quit Twitter, they're going to move to an official client.