The problem with the difficulty decreasing is that it depends on how much and how fast. It depends on how much the tax is relative to the cost to mine. Also, it could also politically be positioned as a "bitcoin tax" instead of a climate change thing to make it bi-partisan. I personally think these are the things that causes Main Street to move on. If all of the commoners sell their BTC the price will drop.
As far as the difficulty stays high enough to keep the network secure the markets don't have a reason to care.