That's like saying a stock price is directly proportional to the p/e ratio. Things have both intrinsic and extrinsic value. You are only considering the intrinsic value. In reality, people mine stuff at a loss all the time because they think it might be work more later, i.e. speculation.
I don't think that's a solid argument. Some coins were structured such that there would be a finite supply (21M in the case of bitcoin IIRC). The health of a coin seems to me to be its transaction volume, and somewhat related its ability to be directly exchanged for physical goods or services.
So back to a previous user's suggestion: instead of buying energy to mine coins at a loss, one could instead buy the coins at a lower price which seems on many levels to make more sense than to continue mining.
Exactly! Which is why we expect the energy expended on mining PoWETH to drop to zero as the coin dies due to lack of interest, which was the upthread point you were arguing against.
It's possible to justify reasons to continue. Bad reasons. But how many miners care?
No, mining is economic activity, not investment. You pay stuff to get stuff. Whether you then invest your profits in crypto has nothing to do with where you got them.
They are not the same concept, in fact they're completely orthogonal. You don't mine to get coins for investment, because you can get coins much (MUCH!) more cheaply via other means. You mine to get income in the present.
Overpaying for an investment doesn't make it NOT an investment. You're just saying that arbitrarily.