I bought WinXP Pro ONE time(for my main PC) and upgraded 3 times... and used the same copy... How many versions of OSX have you had to purchase in that 5+ year time frame to maintain functionality?
To maintain functionality? There are Mac users who are still on OS X.1... and OS X.2... and OS X.3... and they are are fully functional for what they were intended to do. They have not gotten worse. Those who chose not to upgrade still have the full functionality of their release they bought... and then some.