That's not true. Most if not all commercial upgrade versions require some proof of ownership of a previous version...
Which has nothing to do with whether or not you can "start fresh" on your computer. Requiring prior ownership is not the same as requiring prior installation.
Monopoly is a much-abused and little understood term. As a purely economic state, it rarely actually exists. That is why antitrust law addresses issues of market power, which don't require a monopoly to exist.
As a legal matter, they are two separate and distinct concepts, and both very much exist. Antitrust law addresses both market power and monopolization, along with a plethora of other practices constituting unfair competition.
It is simply not the case, as you stated, that Apple must have a monopoly.
Tying one product to another is not an antitrust violation unless the company doing the tying is found to have and be abusing market power.
As a broad oversimplification, yes.
If one product is naturally related to the other, this is generally not considered to be illegal tying. Forcing a consumer to buy one product they don't want along with one they do want can be considered illegal tying.
Yes, it can be, granting a number of assumptions. But that is not to say that it must be. I fail to see what this has to do with what you're responding to.
What's your argument here? In order to use OS X, you must agree to only install it on an Apple labeled computer. That is tying. You can call it a platform prerequisite if you like, but it's still tying.
It's not tying if you're not forced to make a second purchase. Again, this is a distinction of subtlety--if the product is only
offered to a set of people (in this case, Macintosh owners), it's not really tying because your purchase of the product is not barred by an obligation to purchase something else. OS X is simply not offered to the greater market, but instead as an accessory and add-on to their own Macintosh products. You must own an iPod in order to make use of an accessory with a Dock connector--but it's not
tying, it's a prerequisite. Even if we grant the tying status, doing so is not itself illegal, either putatively or in fact. It falls as far, and no further, than promotional tying, which is not against the law.
How can you prove that the retail price of OS X is depressed, and that it isn't the case that the retail price of Microsoft Windows is inflated?
The price of Windows has nothing to do with it. The retail price of OS X is depressed because its sales account for less than 1% of Apple revenue, and yet OS X development constitutes a far greater expense. Investor reports and compliance filings easily show these figures for corporations. OS X cannot sustain itself based on copies sold, and thus it cannot be said to be using its market power in an anticompetitive way.
On the other hand, if it were to
drop the Mac requirement, it would be engaging in unfair competition, viz. predatory pricing.