It's the lack of elasticity of supply.
That creates currency instability.
Cryptocurrency don’t need replace physical money cause its better on its own way, its given that fiat money will never die but this kind of technology will help us to enjoy more in the future. The supply with cryptos is not a problem, because we all know even stock market doesn’t run out of supply its a matter of demand and it will create supply.
Ok some convincing points here. I like the way you separated the crypto currency from the fiat currency. They have no similarities at all and they run in their own different principles. So you see, there is no reason for crypto to go in the banks, in national circulation or to replace the fiat currency in first place. All is vein in front of ever green crypto field.
The supply is unlimited. The coins can gain as much value as they want and apart from that they can be divided into sub tokens too. So everything is manageable in here.