Agree. Even recently, I would have up to 5-6 cans worth myself (have since cut down a lot), but alongside the financial cost, there's also the acid eating away at your stomach lining and the excess caffeine to worry about alongside the Aspartame.
Frankly, given the stomach issues and acid reflux that too much soda can give you, I would imagine that people (even sodaholics) would have to stop much sooner than 20 due to all the other issues involved with sodas before the problems with Aspartame would even come into the picture.
Not to say that I'm not leery of Aspartame, but diet Soda has other major issues beside it.