One thing I have observed with sodas containing aspartame, is the short shelf life, I think normally they give sodas 1 year but after 6 months the soda starts having an off taste that only gets stronger.
I have tried drinking a can of zero that was 2 years past the expiration date, and it tasted like cat piss.
Ps: I guess the aspartame molecules are not very stable in a soda mix?