Well, you suggested prophylactic exposure in early childhood (which is usually before any allergy is diagnosed and usually also before it is manifest):

> I suspect that in twenty years it will be standard to start infants on sublingual immunotherapy for all the common allergens (literally putting a tiny, strictly controlled amount of the allergens under the tongue once a day, simple as that) as a prophylactic

Also, even therapeuthic or diagnostic exposure can cause allergies, there is no safe exposure. Just safer and less safe.