First, I agree with you on the Vista argument. I never upgraded to Vista because there are absolutely ZERO reasons why I need anything more than XP. Most of what I do is in the browser, and my apps are running fine. I have ZERO reasons to upgrade to 7 as well.
However, you lost me on the Blu Ray argument. If you have a 19" TV, no, you are not going to notice, or care, if you are watching a movie in HD or not. But on 50"+, it's so awful watching a regular DVD it completely ruins the experience.
So, back to your response, if you have a small PC with basic needs like coding, website updates, browsing the web, email, upgrading the OS is a waste of time and money. But, if you have a big PC and big needs from your apps, you may have a reason to upgrade.