Asking for a friend... what is the original reason for druids' metal aversion?

[KorvinStarmast]

Wizards aren't prohibited from wearing metal armor, but are prohibited from wearing armor period.

Yes. That goes back to OD&D / three brown books.

I'm pretty sure back in 1E this was purely for balance reasons

Not wholly, since an elf could be a fighter/magic user, but yes. Each role was supposed to have it's own niche. And Divine Casters could wear the armor a Fighting Man could.

But if metal blocked magic, then by that same logic you should wield a spiked chain and wrap that around enemies to keep them from casting.

Sure, why not? Arcane casters are OP anyway. If you broke a round down into segments, some spells didn't go off for a few segments, giving the martials in opposition a chance to disrupt the casting of the spell.

While I certainly appreciate Theodoxus' evocative writing, I can't help but feel that they are a bit more colored with flavor, than facts and citations from the books or the original designers of said books; I mean, they're hinted between the lines, of course, but even I would appreciate seeing solid citations from said books, because, for one I don't own the older books in any form whatsoever (in fact, currently the oldest rules I have at my disposal is only the 3.5 PHB)

The original authors were not subjected to the volume and rate of nitpicking that internet era authors have been. (Althorugh plenty of nit picking went on in various newsletters of magazines like dragon, those didn't flow at the speed of electricity).