Wolfie, there is no reason for anyone to upgrade-but that doesn't do much for the Micro$oft business model.
Microsoft is in cahoots with the hardware manufacterers, and by releasing fatter (more resource intensive) operating systems, they are the driving force behind new PC sales. Micro$oft also forces application developers to make their apps compatible with the new, and phase out support for the old-or face losing their Micro$oft partner designation.
They also stop selling machines with prior operating systems, forcing the corporate market to adopt the new OS.
I could go on and on....