[ClumsyPilot]

If there is a single miner, he/she controls the entire network and movement of money on it. Either he takes everyone's money.

Or everyone abandons a now worthless currency because it just became e-money controlled by a single entitu.

[paulgb]

> and movement of money on it

Not exactly, they could reverse transactions but if they fabricated transactions from thin air other nodes (including exchanges) would not honor them.

I agree with your second point though, the value would tank if this happened because the security of the coins is important to their value.