I say that Microsoft will probably give WP a solid 5 more years. If marketplace penetration stays the same as it is now, they'll tank it like they did Zune. (Note that I didn't mention Kin, because Kin had zero successes, and Zune got a good-sized base of users, many of whom can be found here on WPCentral
)
I don't understand the first part of your statement though... Apple devices already
do outsell Windows devices. iPhone is clearly leading in the phone space (not when compared to Android though), I imagine they sell many more iPads than Surfaces, and manufacturers are terrible at marketing their Windows 8 tablets.
I do think that Windows RT will go down in flames no matter what. I hardly know what it is, and it seems too restrictive. Put Windows 8 on everything and call it a day. Microsoft will be fine as long as it has Windows, Xbox, and Office though. Right now, they just have to keep on keeping on with Windows 8 and pushing forward with what they believe is the future of using a computer and an OS. Windows 8.1 is a start. And then someone needs to light up a fire under the WP team.