While it is true that data accumulates as time goes on, it's not as big of an issue as described. Older data tends to be smaller, for example, photos were lower resolution. That is compounded by the fact that storage is getting cheaper. For example, the price for a 500GB drive in 2006 is the same as a 14TB drive price today. If your getting the same subscription dollars, it suddenly becomes cheaper, every year, to store legacy data.
In my findings, the reason unlimited doesn't work is a small percentage of users (very small, like ~0.1%) who push it to the extreme. In other words, if your exhaust 50% of your resources to service 0.1% of your users, at a fixed subscription cost, the model doesn't work.