That's correct, if the "into anything else" transaction required a cross through (any) blockchain. And the clearing feerate, which correlates to mempool queue

That's correct, if the "into anything else" transaction required a cross through (any) blockchain. And the clearing feerate, which correlates to mempool queue depth, incoming transaction velocity, and cohort feerates, would increase dramatically. But if there was a run to convert BTC to USD, the exchanges would not be able to cover the USD required. They'd limit or suspend withdrawals immediately. At some point someone would have to start buying with fresh USD for the exchanges to be able to pay out the sellers. With big enough news, there could be tens of thousands of dollars gap between bid and ask, which would take a long time to ease.