The change appears to be a part of the broader trend towards flat design from realistic design in the UI community.
A balanced point of view: http://www.webdesignerdepot.com/2013/12/infographic-flat-des...
"Flat design" versus "visual nudging", for want of a better term, is what the debate's really about.
* Skeumorphism is about rendering individual materials. E.g. compare the old and new Safari icons. Both show a blue compass, but the old one is drawn as a realistically rendered 3d-object with perspective and metallic reflections, and the new one is a flat 2d drawing. Similarly, the old design of the maximize/close buttons on windows were rendered skeumorphically, as a some kind of plastic 3d-object, the new ones are 2d.
* Then there is the use of visual metaphors versus abstract symbolism. E.g. the old icon for Photos was a picture of a camera and photo, the new one is an abstract symbol (apparently it's supposed to be a stylized sunflower?).
* And finally he talks about color choices: the old design used lots of saturated colors, the new one has more desaturated ones with a few saturated accents.
All of these can vary independently. A stylized line drawing of a battery is not an instance of skeumorphism, but it is a use of visual metaphor.