Their logic is pretty thin, it appears to be that if people unmask, the virus will spread more (which is debatable on a couple levels, post-vaccinations) and when it spreads more, it might mutate into a new form. But we have no real control over if it will mutate somewhere in the world, it could be in Ghana or something and if it's a genuinely dangerous mutation it would still spread over the whole world. Having a portion of the population in America being masked up more than a year into this thing would probably not do anything.

What's really interesting about this though is the fact that there is no real end to this thing using that kind of logic. The virus will probably still be out there somewhere in some form even years from now and it might mutate. Furthermore, the regular flu might mutate and kill everyone. It just doesn't seem like this reasoning has an end point.

Lastly, if there's one thing we're familiar with, the left has an issue with admitting when they are wrong about something. The more they resist unmasking, the more they will become unwilling to admit that masks aren't doing anything, seems the most likely outcome to me.