They took it on themselves. I believe that they should at least let use have the upgrades for free. But I believe that they should only give it to people if they have Windows 98 and have registered it with Microsoft to prove that they didn't illegaly get it. Of cause, they make life easier with more fancy desktop and menu bar. But for functionality, you can see the trend.
Compatibility always goes opposite to stability.
NT is the most stable one as it is not compatible with lots of software. It still is on the first day when there is no software in it. For XP, how can it be incompatible with both the 9x version and NT version of Norton Anti-Virus. I would expect that it can be compatible with one of the two. Such as linux but doing this is just hurting their products.