Nah again, this has nothing to do with Microsoft. I'll leave Apple out, too.
The "common opinion that" is nothing but hype. The thing is, "PC" market has plateaued; it's a mature market. Tablets, phones & other gadgets are not "killing" it, they are growing their own market, mostly for the following 2 reasons:
1) 91.23% of population doesn't need more than email, photo/video, web & simple games.
2) People who spent money upgrading their "PC"s every so and so (for values of so and so ~ 1-2 years) do it far less frequently and they can spend that money on other stuff.
Which leaves us with the elephant in the room in these kind of stories, which is business. Business predominantly buys boxes and will continue to do so. There is no comparison to 80's "PC" revolution, as the boxes are in the same ballpark price wise as the hyped gadgetry. There is just no reason for anyone to go through hoops so they can format-shift their business from Wintel/Lintel to whatever.
In a couple of years, when everyone and their grandmother owns a good enough phone/tablet/whatever, we'll have another round of "X is collapsing".