Masking doesn’t render the air “devoid of pathogens”, it just reduces the amount of pathogens you’re spraying into the environment while you’re contagious, and provides some filtering of the air you’re breathing through your mouth and nose.

Our immune systems will still get “exercised” by fighting off the pathogens we do encounter, but they won’t necessarily be so easily overwhelmed since we won’t be constantly inhaling more and more virus while our lymphatic system is busy fighting off the replicating viral bodies that have already gotten in.