Not A Monopoly.
Apple is a hardware company. That's where it's money's at.
The software, including OS X, is a vehicle to sell hardware. The same goes for App Store drives iPhone & Touch sales.
This is also why OS X and other Apple software such as iWork is so much cheaper than MS equivalents. Sell the software cheap, and the hardware at a premium.
Can you have a monopoly on your own product? No. You invented it. It's yours. If you can protect it, then you should.