What about a few cans a week?
If you’re worried about it causing weight gain and/or metabolic syndrome, one serving a day is more than enough to be dangerous.
Nobody apart from maybe sales folks ever claimed these things are safe to drink in such amounts regularly. Just like nobody ever claimed that consuming say 100g of pure salt every day would be OK, yet you can buy endless amounts of 1kg bags if you want.
Do folks really need some government agency to come to them personally to tell them how to live their lives, babysit every single decision they make, steer them away from their addictions on sugar etc? That's not how game of life works.
Isn't the point that diet coke doesn't have calories[1]? This is why it uses aspartame as an artificial sweetener.