As a currency, they already have. Transaction fees and ridiculous instability are common to every cryptocurrency that becomes popular. These are also the reason why most vendors will never, ever want to support it. Steam just dropped support for Bitcoin and the same will happen with any cryptocurrency. Because a currency without a central authority that keeps its value stable and predictable is a useless currency. Not to mention the fact that every major cryptocurrency is deflationary by its very nature and deflation is universally considered awful for economies because it encourages people to horde money. It isn't important because it is a complete non-starter as an actual, functioning currency.