I thought the world had gotten beyond "Steve would be rolling in his grave". Apart from anything, Windows Phone and its flat design came out in 2010, three years prior to iOS 7. Windows 8 came out in 2012, one year before. Please don't pin an entire phenomenon on one man, especially the wrong one.
I don't think Apple copied the flat UI from Windows Phone. It had already become a trend by that point, and OS X had started to move to lighter gradients and flatter buttons in 10.7, released in 2011. I'm sure the web had already started getting flat by then, too.
