Systems are supposed to get MORE stable as they age, not get worse or show no improvement over time.
No, it is basic knowledge that software becomes less maintainable as it is updated and eventually a change takes more time than a rewrite.
The earlier code is eventually based on assumptions that the later code is inconsistent with.
However, if the feature set is static, your statement is correct. The computer manufacturers and hence the system software developers such as Microsoft are under pressure to sell new system and therefore add to the feature sets constantly.
Then there's the question of competence. XP handles dual processors adequately. Apparently Vista has some issues. oops. The DRM is enough reason for me to pull any disk on a Vista machine and install a fresh one to run Linux on. Windows 7 takes a fresh approach to multiple cores but to do it Microsoft had to replace much of the kernel code and then work outwards to ensure compatibility.
There's still the DRM. I don't download music or copyrighted files but I won't pay for someone to tell me I can't even have my fair use rights. In this case, it's the functional requirements themselves that have made the system worse.