And nothing really stops exchanges grouping together and starting to do things with fractional reserves. It is all digital anyway, so blocking the few big askers from taking their coins out with various reasons wouldn't be big thing. And average user wouldn't really care how their money works...
Fractional reserve is already a thing and will only increase from here. Bitcoin's scarcity relates to the base money supply and obviously cannot coerce the higher economic layers.
That said bitcoiners are somewhat resistant to extreme fractional reserve because of the strong "not your keys, not your coins" ethos.
