My stuff is still white but going into second-hand shops most white consoles have turned yelllow. The average consumer mistreats stuff esp in the US. A gaming forum crowd lives in a bubble.
My dad isn't part of a gaming forum and his white consoles and tech things are all still white... maybe people should just learn the absolute basics of looking after their things?
I worked in second-hand shops and managed 1 for some years. I also game hunt a lot and finding a white system is rarer. Japanese shops it's usually better. Except it's a mixed bag on Super Famicoms. Heat and/or UV light reacts to a chemical in the plastic. White Saturns pop up a lot but every so often a yellow one creeps in. In the US tho games and systems are heavily abused. Missing inserts. Cases. Scratched discs. Yellowed systems abundant. Oddly one thing I saw a lot is Wii's were fine except the panel for Gamecube controllers turned yellow. Just the panel door. Then I don't even want to touch a smoker's system. That smell never gets out. One common issue with used consoles is dead bugs inside them. Cockroaches esp. The plastic yellowing mainly depends on how much heat or UV light it gets.
Somewhat unrelated but another issue with older electronics is those that had a rubberized surface. It deteriorates after a decade or so into a sticky gooey mess. An MS Zune from 2007 or some older Laptops or an OQO.