> I give it 50 years max before possessive "who's" is considered correct (along with "it's").

"It's" is one I've struggled with a lot. I understand "It's" -> "It is" but my brain wants to add an "'s" for possessive-ness. It just feels more right. I'm been able to mostly break that bad habit but I still don't like it.

It's a strange rule. I'd be interested if any more serious grammarians can explain where this irregularity comes from.