Do you have any evidence for the statement that 1 can every two days is OK, but ~7 cans every day is not OK (other than anecdotal)?
The kind of person that'll happily drink a bucket of soda (diet or otherwise) is likely the kind of person that has an unhealthy relationship with food in the first place, with a stomach (appetite?) capable of ingesting a lot of food and liquid. Which is a category morbidly obese people fall under.