If BTCUSD Price Drops Far Enough Are Transaction Fees Enough Incentive for Miners?
Wouldn't the transaction fees for mining simply go up to cover the cost of mining?


No, it wouldn't. Transaction fees are not related to the exchange rate. They are only related to the supply and demand of available block space. If not many people are transacting and blocks are not full, then there is an excess supply of block space and not a lot of demand for it. This will drive fees down, regardless of the exchange rate.

If the exchange rate were low enough that the block reward is not sufficient (regardless of the block subsidy existing), miners would simply stop mining. This will eventually reduce the cost of mining such that those who are still mining will become profitable again.