What Will Exchanges Do When Supply Shock Hits?

I mean, do we really expect them to all of the sudden not hand out IOU's? "Sorry guys, we're all out. Check back later... maybe..."

No, but they can't give out IOU's forever. Or they can, and just prohibit withdrawals?

I know the last bitcoin won't be mined for another hundred something years. But supply shock should theoretically hit long before then, in which case the demand for btc will immensely outpace the mining rate/newly made available btc.