in 21st century colloquial speech, a linguist would have to admit that, descriptively, “widely applicable” and “allegorical” are nearly synonymous

Ha. You're the second person to have suggested that, so maybe there is something to it. But to be honest I'm not sure I agree. I don't think I'd ever use the term allegory without authorial intent. (But to save repeating myself, I'll just direct you to my reply to @dragonfucker@lemmy.nz.)

Or, at the very least, even if you are inclined to disregard authorial intent, there's still a subtle difference between allegory and applicability in that allegory requires an almost direct one-to-one relationship between the text and various elements of the real world, while applicability can be much more subtle or broad strokes. Basically, applicability is a broader term than allegory, a superset.