Proponents of cryptocurrencies could easily fix this by creating one that grows by k% forever (like Friedman's k-percent rule). But they won't because this destroys the "get rich quick" aspect of the technology which they won't admit is the main thing they are drawn to.
If they were serious they would have fixed this problem and made sure bitcoins generate long run inflation.
Could bitcoin's popularity stand on its own with just its net benefits as a transaction medium after the allure of ponzi investment has been removed?