Maybe. I doubt most consumers of sugar free soda are drinking more than 4 (which is already a lot). I have to imagine that, like most things, most people consume them in moderation and have no ill effects.
That said, I have to imagine if you go from drinking ten sugared sodas a day to ten diet sodas a day, your life will change in a very positive way. That would be removing 1500 calories of pure sugar from your diet and that's gotta change people's lives.
The small Coke Zero cans make it tough to have just one. I typically will have 2 small cans with a meal which feels roughly equivalent to one regular sized can
Except that four cans of soda is not much more than a single 44oz soda fountain drink at QT and folk gobble down those often 3 times a day.
I lost a lot of weight using diet soda in the evenings as an alternative to snacking.
It turns out, drinking a lot of what is essentially just water, is actually pretty healthy.