If the coin's point is to become valuable to the point that people want it to trade it, then a PoW coin does have to require more overall compute power to crunch the numbers on the ever-increasing amount of transactions that might be attempting to double-spend on the chain. This is all because of PoW's lack of focus on efficiency when there is that much demand and that many transactions happening at once. Electricity, hardware, and profitability are all externalities that are the result by the amount of usage of a PoW coin, not the cause of it.

> When a coin spikes in value, the profitability goes up so more miners join in. A single PC would have enough processing power to run the bitcoin network.

well, yes, but then a single miner could and would control the entire network since 'why would any give up their compute power for free to ensure the network is not being hit by double-spend attacks, when someone else is already doing it'. The whole point of a PoW coin having huge fees and thus returns for the miners is to incentivize a decentralized ledger. I don't think Satoshi intended for it to eventually consolidate into just a few huge mining companies which theoretically could perform a double-spend attack, but it's not something that could have been protected against with a few more if statements (although, if this happened, the coin would tank in value, which is probably the inherit safeguard).