Just because we can’t guarantee a perfectly competitive market doesn’t mean the government can’t try to ensure one. E.g. you mention Linux as a competitor to windows, yet the government itself is a huge buyer of windows and Microsoft products in general. A role of government should be setting up and ensuring as close to perfectly competitive markets as possible.
"A role of government should be setting up and ensuring as close to perfectly competitive markets as piasible.[sic]"
Your argument is not that different from people who say things like "we don't have perfect competition, that is a market failure, the government must fix it". As I said before, perfect competition is a model. It isn't some utopian ideal. The argument as I phrase it is basically the Nirvana fallacy, and I don't think I'm mischaracterizing your views.
I would be more sympathetic to arguments like: "anti-competitive corporate behavior, like the formation of monopolies or cartels or other means that reduce output and raises prices, is not socially optimal. The government should prevent such behavior"
In other words, I think you adopt a position that tries to prove too much. This merger may be bad (or it may be good, I don't really know), but you don't have to rely on the argument that if competition isn't perfect then the government should step in in order to oppose it. That's not a good argument.
While I agree that governments shouldn't buy Microsoft products, it's not really a competition when nobody there aren't any companies trying to develop or sell linux as a client os for end-users.
