> What if cryptocurrencies do not have the inflation problem?

Then it is rational to hold, not spend, them. Deflationary currencies don’t function. (They can be stores of value.)

But inflation isn’t a problem, it’s a benefit. We want people to spend or invest (in production) their money; economy works best when money is actively circulated, not hoarded. The problem with a totally fixed money supply is that as productivity grows new investments are disincentivizes, as a fixed currently holder get the benefits of aggregate productivity increases (through deflation) without risking his money in investing in companies.