2 litres of (diet) soda is definitely an absurd amount

2 liters of soda everyday is huge. If it was sometimes (in a party, etc) it would be fine, but I can't imagine drinking close to that amount everyday.

I am sure that people do even worse, but at that point you're in a bad situation if you drink normal (e.g.: the amount of sugar) or diet soda.

Daily recommend fluid intake for men is 3.7 liters (20% from food).

2l of something doesn't seem excessive.

2L of what is essentially a dessert liquid is very much absurd.

We're not talking about water.

2L of pretty much anything that isn't water is very likely too much.

Put it this way: go read up experiences on reddit of people doing the GOMAD diet.