Remember there's only going to be a finite number of Bitcoins ever created, so they're a limited resource! Also, there will be a finite amount of Ripple, and a finite amount of Ethereum, and a finite amount of Bitcoin Cash, and a finite amount of Litecoin, and...
They will continue to increase in value relative to supply (how many are being created) and demand (how valuable merkins believe they are). Because they have no inherent value over other currencies (except that criminals prefer to be paid in them) and are actually more volatile because no government has any control over their price, there will eventually be a slowdown as people get over the hype and FOMO dissipates.