The entire argument behind capitalism is that high margins attract competition that drives those margins down towards zero. If that isn’t happening, it’s due to an environment that isn’t competitive (e.g. too high a barrier to entry.)
In the real world the technology industry does tend to have higher margins than other industries. There might be perfectly normal explanations for that, such as network effects, but there are also government policies that have the effect of reducing competition. For instance, intellectual property laws reduce competition in order to attempt to encourage innovation. The strawman version of capitalism doesn't exist in the real world. Margins can remain high for some time.
That being said, there are competitors for Microsoft's bread and butter products. If you want an alternative to Windows, try Linux. If you want an alternative to Office, try Open Office. For many users, however, they get a better experience with the Microsoft products than these alternatives, even though they are free. Microsoft has to keep making their products better than the alternatives or people will use others (though there are costs of switching and network effects that mean that MS probably doesn't need to have the absolute best product on the market in order for customers to keep using them).
Unless you go out of your way to buy cheap laptops the difference between macbooks and your favorite dell business longitude isn't as bad as you think.
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.