Not so much where the profit comes from selling software.
If Microsoft open sourced Windows and Office they would instantly stop making money, hence they will always have an uncomfortable relationship with open source.
> If Microsoft open sourced Windows and Office they would instantly stop making money
If Microsoft open-sourced Windows and Office and offered enterprise technical support contracts at the same price they offered enterprise licensing contracts for the same level of support that was included in those contracts, they'd probably keep making money, though perhaps somewhat less (they'd probably decrease costs, though, as well, since both the administrative side of license management and building license enforcement mechanisms into all your software has a cost.)
> If Microsoft open sourced Windows and Office they would instantly stop making money
It's funny because I don't actually think this is the case. Not completely, anyway. That's their fear, but I think that the upside in the end would counteract the immediate downside.
It could also be structured interestingly- Basic Windows could be free, but certain diffs/patches/drivers could be for-pay (such as higher-performant stuff, graphics stuff etc.) A mix of free OS and proprietary profitable stuff. They could also employ an army of Windows consultants...
If Microsoft open sourced Windows and Office they would instantly stop making money, hence they will always have an uncomfortable relationship with open source.