I think your point is interesting, but I have disagreed previously with someone else using a similar analogy.
For one, you've arbitrarily decided that 18 months is the de facto "support" period for a phone to be kept up to date. Why is this? You're apparently basing this so-called reasonable amount of time on the rate of technology advancement within the smartphone sector. You seem to be neglecting the fact that carriers who subsidise handsets, particularly in Europe, have contract lengths of between 18 and 24 months.
Let's also consider that Windows Updates usually GIVE you 10 years worth of bug fixes, for absolutely free. Whereas with an Android device, you're saying it's fair to pay a monthly subscription fee but then be left out in the cold after a time that you've decided is reasonable. No, a consumer shouldn't expect a brand new version of Android, but fact is a lot of these are point releases. They fix BUGS and PROBLEMS and ISSUES. Not unlike a patch. You shouldn't have to pay for a patch, and so why is it not reasonable for people to expect they get a patch?
Your analogy holds no water at all. Desktop operating systems are kept up to date for free for the support lifetime of the operating system (which with Windows is usually a decade or something) for a one off payment for the operating system itself. With a smartphone you're at the mercy of the carrier and manufacturer irrespective of whether you're paying a monthly fee or you bought it outright. It's completely different.