Here's why MS'plan would never work....
First let me kill one particular misconception: MacOS X is *not* a walled garden. If you think it is, you're thinking of iOS, and even that is pushing it, because people have jailbroken that. UNIX-based OS's (like MacOS X and iOS) cannot be a walled garden.
Now the reasons why the "forced Windows" will never come to pass:
1) Windows 8 will tank, harder than either Vista or ME, and it will be for the same reasons Office 2010 is still being eschewed in favor of Office 2007. Their 'ribbon' menu system is so vastly different than what people are currently used to. This is NOT what you want to force on less-computer-literate users who are used to the current "File Edit View" hierarchy. As for the "tiles" interface, it's not working for Apple (how many do you know actually use the "Launcher" as the main interface), so what makes MS think it will work for them on a desktop OS?
2) MS would have to make this LAW to succeed at it, and they would have to do it WORLDWIDE. Good luck with that, and good luck with the lawsuits from Apple and Google (both of whom have far better legal resources than MS) that would result from even TRYING to push this through law. Why would they sue? Because such a law would also illegalize MacOS X (UNIX), iOS (UNIX), Chrome (Linux), and Android (Linux)...not just the smaller UNIXes and Linuxes.
3) Because they would never be able to pass this worldwide, there would result in a grey-market for computers that can run other OS's, yet identify to internet routers as Windows to allow them online, completely killing the effectiveness of their plan.
There are simply way too many ways to keep MS from doing this. Their plan to lock everyone into Windows is a cry for desperation from a company that is hemorrhaging money. At this current rate, I don't see MS as being a viable company past the end of the decade. Look in comparison: MS tanked HARD yesterday in the stock market (about $1/share, but that's still about 4-5% - probably over the news that Bing lost them $5.5 billion over the last year), while Apple is posting its highest numbers EVER... *without* Steve as CEO! Wall Street knows where the companies are heading already.