To me, this is why Proof of Stake is the only long-term sensible decentralized cryptocurrency system. ISTM PoW and PoST ultimately equate to the same thing, for the reason you've suggested. If it's profitable to invest more in mining, people will invest more in mining; it doesn't matter whether that investment takes the form of ASICs, GPUs, HDDs, or simply the opportunity cost of holding the currency itself. The only difference is, that last one doesn't burn a bunch of real world resources in the process.