Every time they bump into something they dislike about Chrome, they’ll remember who is forcing them to use it. When an alternative comes along and works in their browser of choice, they’ll switch and advocate for it.

We don't know how the situation will look like in the near future. If a large enough portion of the users switch to a different browser, it quickly becomes an issue.

It is in my opinion part of what companies should include in their risk assessments. Looking at the general trends, I would not be surprised if at least EU governments and institutions will ban Chrome due to privacy and data security. That will probably lead to others switching as well.

Ideally, you'd have reliable usage numbers by browser and platform, as well as a threshold of usage that demands testing. Maybe that's 1%, 5%, 10%, 20%? Aggregate up, so if FF desktop doesn't hit the numbers, maybe FF in general does, or maybe 'other browsers' does. Or maybe commit to testing the top N platforms over 1% usage.

Lower on the usage scale doesn't need to be pixel perfect or pretty at all, IMHO, but major site features should still work.