First let's understand what 7's target is. It's not so much desktop Linux as a particular Linux distro -- Ubuntu -- that targets the desktop.
Some of these reasons are technological, others social. But the biggest reason is a business reason:
Ubuntu has an entrepreneur at the helm. Microsoft beat IBM because it had Bill Gates up against a bunch of suits. Now Microsoft is a bunch of suits and Ubuntu has Mark Shuttleworth.
Ubuntu has more server compatibility. Linux continues to beat Windows on the server, and servers (in the form of clouds) are becoming dominant over clients.
Ubuntu's friends will not desert it. HP and Dell have gotten a taste of freedom from Microsoft tyranny. They won't give that up easily. They will continue seeking product line niches where Ubuntu can succeed.
The Netbook will continue to evolve. The "no moving parts" PC is still at Version 1.0. There is a niche for a cheap, profitable "online machine" that can be used in Airports, hotel rooms and sandy deserts.
Applications will come. It's true there are more Windows apps than Linux, by a logarithmic factor. But many are being ported, and the developed world will create many more.
Windows is losing the mobility wars. Linux is playing well, Apple is winning big, RIM is hanging in. Where's Windows? PCs are servers to mobile clients. The clients will tire of waiting.
The open source model. Software development has become like the old Steve Jackson game Ogre, and Microsoft's the Ogre. Ubuntu and other open source pieces are small, but there are a lot of them, and they can work together. Or as Kermit said in The Muppet Movie "who are your friends, Doc?"
This does not mean that Microsoft will cease to be a desktop player, that Ubuntu is going to take over. Far from it. Ubuntu will retain enough interest to stay in the game, that's all it needs to do.
Ubuntu is doing fine with its present market share. How much more can it get over the next year? Perhaps just a little. I suspect this will be a year when desktop Linux consolidates around Ubuntu, so some gain is nearly assured.
Microsoft will also not disappear under the open source onslaught, just as IBM did not disappear under the weight of Microsoft. But it will change.
It will be fun to watch how it changes, and when. It has yet to see the need, but when Ubuntu fails to fall under the weight of Windows 7 perhaps it will.