- Storage price decreases near exponentially, while data usage also increases near exponentially, meaning that new data will always make the majority of your usage, no matter how long you and your users have been in place.
- On Google scale, your are bound to have a lot of redundancy in data. Chances are that what you have in your Google drive, someone else has it. And large data provider often only store one copy of the data internally (with necessary backup). It is controversial for privacy reasons, but that's a way to deal with a large number of users using a lot of data.
- There is only so much a user can produce. For example, if you are dealing with eBooks, you can probably put all text only eBooks humanity have produce on a single hard drive. So no problem going unlimited here. For pictures, unless you are constantly shooting, you are not going to produce terabytes of data. 100 GB is actually an awful lot of data, and few applications produce that much, for most people, only video is on that scale.
So it means there are natural limits. Also, unlike with a Ponzi scheme, there is actual value being produced. People pay money for a service, they don't intend to get their money back. There is no real reason for the system to collapse.