Though 300 stores is a bit unrealistic, the stores still need to exist for the next 40 years or so at which point there will be no point since everyone in North America will know how to use a computer anyways.
The only real boxed product that seems to be sold in Microsoft stores is, well, Windows and Office- both of which you can get online. Any other developer tool that you would buy there is also downloadable.
The real attraction is, really, to buy a computer.
All needed software? Already installed.
All crapware? Removed.
All stickers and other traces of OEMitis? Removed.
Would I suggest to someone to buy a computer from there for a 25 to 50 dollar premium over the same computer in Best Buy?
Of course I would- if I couldn't do that work right away.
Also, classes on "How to use a computer 101" are the second main attraction. Office use? Covered. How to perform basic maintenance tasks? Covered. How to not screw up your computer? Covered.
Xbox 360's? Unless you're selling games I see no point in getting one from a Microsoft Store vs. a gaming store where you can pick up games (you know, the things you play on an Xbox 360?) right away.
Microsoft Surfaces are apparently pretty cool but unless you have some for sale, well...
The issue is that the youth market is lost.
For smart people, it's Windows, and these people don't need the store since they already know how to use a computer to buy things online and know how to fix one when it 'breaks'. These people have also been around computers enough to know how to get tasks done and don't need a tech to ask questions.
For hip people, it's Apple. These people don't care to know how to use a computer and need a tech to whine to when the machine breaks/see something they don't want to learn to use.
That, incidentally, is why Apple makes a killing on margins since the hip user can make do with the crappiest machine possible in terms of hardware specifications as long as it looks good- it never appears slow to the user since the user never does any real work on one anyways: and Office applications aren't slow anyways.
The issue, though, is that those who don't know how to use a computer and physically cannot learn how need this help. And they've been around long enough to know that Apple isn't financially smart for them. So, Windows is it- and they need the services that the Microsoft store provides. They need a crap-free computer that runs perfectly with everything they need, and they also need to know who to ask when they need help when it breaks.