I actually agree with you, in that any sufficiently dominant corporation is indistinguishable from a government.
However, even if you set aside the question of its dependence on government-enforced IP rights, Windows is a bad example of a harmful monopoly, IMHO. Network effects make the calculation of net "harm" very difficult. Thousands and thousands of entrepreneurs (to say nothing of Microsoft employees) have become millionaires thanks to those network effects. Good things have happened in the world of personal computing that could never have happened in a fragmented world where dozens of vendors were pushing their own 6502 or Z80 boxes with proprietary BIOSes and OSes, or where Linux geeks worked day and night to make sure ordinary users would never be able to accomplishing anything on their own. Somebody had to step in and harness all of those creative forces in a productive direction. Frankly, I'm glad it was Bill Gates and not Steve Jobs.
Where I think you are wrong is with your supposition that "IBM mercenaries" would have roamed around like mafiosi, crushing the PC revolution. That's what I call moving the goalposts, or more properly, a strawman scenario. You won the argument in your own head by inventing a fictional world where you could claim to be right.
Remove government, and businesses would develop their own systems like copyright and patents, which they would privately enforce through contract, private prisons, and mercenaries.
I'd need to see an example to buy this. (Don't use Somalia or any other failed Islamic states dominated by other governments' proxy warriors.) We don't live on the set of Blade Runner, at least not yet.
I guess the closest example I can think of would be the medieval guild systems. Those aren't coming back, not as long as capital can flow freely between nations and individual actors are gaining rather than losing influence.
You replace one system with another that's almost identical, but small businesses lose any kind of recourse if a bigger business decides it owns a particular creation.
The patent system is doing a better job at that than your violin-case-toting IBM thugs ever could have.
The system where that's real is as real as whatever free market system exists in your head. I don't think football metaphors (goal posts) will work until we agree on what the stadium looks like.
Until that happens, all we can do is talk past each other based on our own ideas about what "free market" means. A free market to me is one where the strongest invariably crushes anyone weaker. Anything more restrictive would require government or cultural norms. Moving to any more or less restrictive system requires changing at least one of those things. Any discussion that doesn't see that isn't going to produce anything practical.
And I'm not interested in discussing hypothetical markets, so I'm going to go do something else and ignore this subtree.