There seems to be this idea that angry developers -> no apps -> crappier service -> users leaving, and I just don't see it. Apple has long been screwing developers with their App Store and yet how many thousands are out there coding away at their iPhone games, playing by every rule, however arbitrary, that Apple implements?
Developers aren't going to kill Twitter, but a better service more attractive to users will. I'm of the belief that at this point, Twitter's developed such a huge brand that the lack of app choices alone is nowhere near enough to decrease user engagement. Twitter is constantly on CNN, a myriad of other news channels, and almost every major celebrity tweet I've seen has been made from either the Twitter web interface or their Mac/iOS app.
Users have a much, much larger threshold of "abuse" than many people here seem to believe. Facebook ads, Twitter ads, no third-party Twitter apps, etc, are all very minor annoyances, if you can even call them that, to the majority of users. So while Twitter's new move is certainly frustrating for the developers that helped give them a boost initially, they've certainly got the momentum, brand-recognition, and celebrity engagement to keep them going for a while.