Or even simpler: if you put barriers in the way of users cashing out their "earnings", you can keep a naked pyramid scheme running for quite a while. Some schemes have done this by making it difficult for users to cash out their earnings, e.g. by enforcing a lockup period, or requiring users to sell their tokens to another new investor, or even by simply declaring that withdrawals are a feature "under development".

One notable example of such a scheme was OneCoin (https://en.wikipedia.org/wiki/OneCoin), which wasn't even based on a blockchain.