I completely reject that anyone who would have paid $x for a windows upgrade has then decided not to because of the price of OSX upgrade. It's either worth it for them, or not. Lowering the price simply increases the number in the first category.
And your forced car analogy is seriously flawed.
The analogy went some way to showing how the argument based on there being "no direct competition" is a fallacy.
I find it very hard to believe that MS hasn't had a good look at Apple's OS X pricing strategy before making this move. It mightn't be that they feel pressured into lowering the price so much as they've just seen what Apple has done and think it's a good idea to get as many people to upgrade as possible.
If you've ever owned a Mercedes, Volvo, or Porsche,* I don't think that you would find the logic underpinning the auto parts analogy particularly plausible.
*912, 914, 924, and first generation 944 parts sourced from VW excepted.
If Porsche were charging less for parts than Chevy...?
Changing from an automatic car to a manual may save some money when you buy your next car - if you want to learn manual.
For many people, changing OS is probably similar to learning manual when they're happy driving automatic.
I can't use Chevy parts in my Honda, but if Chevy started charging 1/10th of what Honda is charging for similar parts, Honda would need to lower prices if they wanted me to stick with them for my next car, or to use genuine Honda parts instead of third party parts to repair my current Honda. That's because Chevy's price would help recalibrate my expectation as to what kind of prices are reasonable, making Honda look overpriced.
OSes aren't as interchangeable as cars, but a Microsoft upgrade price significantly higher than Apple's upgrade price might convince more people to stick with what they have until it is time to buy a new PC and get Windows 8 bundled, rather than buying an upgrade, because Apple's prices lower the expectation of what an OS upgrade should cost.