We learned a lot about using 5.1 for XP and how that helped developers with version checking for API compatibility. We also had the lesson reinforced when we applied the version number in the Windows Vista code as Windows 6.0-- that changing basic version numbers can cause application compatibility issues.
So we decided to ship the Windows 7 code as Windows 6.1 - which is what you will see in the actual version of the product in cmd.exe or computer properties.
http://windowsteamblog.com/blogs/windowsvista/archive/2008/10/14/why-7.aspx
...as a post-graduate he was told he didn't have much longer to live...
Let this be a lesson to all - graduating is hazardous to your health!
If someone was trying to install XP, but didn't have a disk and asked "Well, I got a Windows 95 CD, shouldn't I be able to use that and just update?", they would probably get a lot of weird looks... but the appropriate response would be "No, it isn't like Linux".
That is not solely a Windows problem, you are comparing a commercial product to a free product. The same could be said about upgrading from MacOS 9 to MacOS X.
"The four building blocks of the universe are fire, water, gravel and vinyl." -- Dave Barry