Second, I don't think OS X is the real threat to Microsoft. Sure, the Mac still grows in popularity, but it's relatively marginal. The biggest threat to Microsoft is the popularity of tablets. The iPad convinced people that they can use it for the majority of their home computing. Every time the average consumer buys an iPad it's a lost or delayed PC sale. Imagine what happens when Google gets things right, and the tablet market explodes like the smartphone market before.
They could do it the same way Apple prevents you from installing OS X on commodity hardware: http://en.wikipedia.org/wiki/OSx86#Legal_issues_and_Apple_ob...
When I heard that iPhones back in 2008 (and later Androids) were going to be citizens in the Exchange world, I did a double-take... in BillG's day this would've never have been allowed.
Full-on Post-PC without Exchange (especially 4 years ago) would have been a tougher sale.
b) Yanking support breaks their customer's trust; and could possibly invite more anti-trust attention.
Do you really think people will ditch their Macs and go back to Windows?