The key is to make data a liability not an asset. So it is too expensive to store any data that is not directly essential. Maybe a ratio of number of users in y

The key is to make data a liability not an asset. So it is too expensive to store any data that is not directly essential. Maybe a ratio of number of users in your database to the turnover of the company? A flat fee of $100/year perhaps for every permanent record you keep, a company could still sell products perfectly well without requiring users to actually register.