I wouldn't hold my breath on that. They've permanently lost several key non-PC markets (tablets, smartphones...) and they're losing the PC market irretrievably portion after portion. They can never hope to break into the tablet market not with the prices functional, usable Android tablets are selling at. The original Nexus 7 can be gotten for as little as %50. Same goes for smartphones. Why should a buyer break compatibility even with one app when Android devices are already selling for the lowest possible prices?
2-in-1 Convertibles have been a dud, as they should be: they're way too gimmicky to replace proper clamshells and way too expensive to replace tablets.
Touch-enabled laptops have also been selling poorly. So what other vector of attack is going to allow Microsoft to break into the mobile market? Not to mention that it's harder to convince the market to switch to a more closed platform even if it didn't have such as massive drought of apps.
The low end / casual "everyday user" they'll probably lose ChromeOS and/or Android. And even if they don't it means margins in that category will be more razor-thin than they have ever been.
On the high-end, there's gaming and creative productivity.
In gaming, Mantle simply shipping means that Microsoft can no longer lock-in half the Gaming/GPU market. If they try any anti-consumer shenanigans with DirectX, AMD users won't be affected and devs will start including the tag "runs best on AMD GPUs" on their games. Nvidia would have to either (1) adopt Mantle (or develop their own equivalent API) or (2) get left behind.
All that's not to mention SteamOS whose victory is all but assured given the unique selling proposition and value it delivers to every component of the gaming industry (developers, store owners, modders, gamers, chip makers...) and simply due to the fact that the whole industry has finally realized that Microsoft has been the worse possible custodian of the PC gaming industry and going forward, it's not going to work anymore and only a shared, open source, industry standard platform such as SteamOS will do.
As for the creative productivity market, there's no top-shelf professional program that Linux doesn't have that Mac also doesn't. The creative pro market is already shared between the 3 OSes.