Frankly, I cynically suspect that the flat, semi-functional design became widespread when the revenue model of many computer and phone vendors started overwhelmingly relying on super-revolutionary "apps" being churned out on minimal budgets.
Today, you can get a complete design for the kind of money (and time) that got you a really good icon for an OS X application back in 2005. It's a major step back in terms of interaction model, not only looks, but the consumer industry runs on money, not quality.
I doubt the plausibility of that timeline - the original iPhone was announced in January 2007, KDE 4.0Alpha1 was released in May 2007[1].
KDE 4.0 was a terrible release (missing functionality, lots of regressions). IMO, they should have kept it as a develop branch and only released 4.1 as 4.0.
1. https://www.kde.org/announcements/announce-4.0-alpha1.php
What's really funny and sad is that the GNOME team made the exact same mistake with Gnome3.