Knows that "while" was once a noun in the good ol' days when English was still declined.
True - and, as far as I can tell, the use of while as in "While I was mowing the lawn, the cat was chewing the power cord" didn't exist. (You'd have to use the Old English form of "when", which was "ða" (pronounced "thah"), as in "When I was..." etc.) Bear in mind, though, that "while" does still work as a noun, as in "It was a while before I could find the charred remains of the kitty."
I actually have a theory about that.

"While" looks and sounds remarkably like the German subjunction "Weil", meaning "Because". As "While" is now largely a subjunction too, albeit a synchronous temporal one rather than a causal one, I suspect the terms are related. Similarly, "eine Weile" in German is often translated as "a spell or period of time" and over the generations it gradually evolved into meaning "during".