Germicidal UV could make airborne diseases as rare as those carried by water

Idiotic and massively overlooks/underestimates how complex biology is.

What about beneficial and neutral but important bacteria and viruses? "Air" is actually a complex soup of all types of things. This like applying HCl to a skin infection.

I doubt it would be a problem for the microbiome [1] but I would worry about the immune system. Would being inside in sterilized air all the time mean you can't go outside or into a forest without getting really sick?

[1] but who am I, it would still be worthwhile to check obviously

That's not how the immune system works. Practice lets it learn how to fight specific bugs, it doesn't make it stronger overall.