[paulgb]

> That's assuming that the mining hardware doesn't get more power efficient

If mining gets more power efficient, holding everything else the same, the difficulty will go up until the efficiency improvement is negated. Bitcoin is designed such that efficiency improvements are eaten up; otherwise every time there was en efficiency improvement it would become cheaper to attack the network.

> I think there may be a future where energy is no longer the deciding factor in profitability vs the hardware itself and operational costs (land, employees etc).

What would the catalyst be? If anything I see this going in the opposite directions: the more money at stake in mining, the more it makes sense to make big, efficiency-improving investments that take upfront capital but are amortized over time.

The one exception to this would be if there were a truly breakthrough improvement in hashing technology that was captured by a single miner, in which case that miner could essentially force everyone out of the market by pushing the difficulty above everyone else's break-even point.

[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.