What you said is not true in case of PoW, and we know it experimentally already - mining costs follow the price of bitcoin, not the other way around. You can look at the charts yourself.
It makes no sense to try to mine a crypto token if the cost to produce one is greater than its value. Advances in mining technology or the increasing value of crypto can help keep it profitable, but there are diminishing returns to investment in technology, and (one would hope) the value of Bitcoin will eventually stabilize.