ONE REASON MICROSOFT MADE MEDIOCRE OPERATING SYSTEMS

It has a financial incentive to get consumers & businesses to upgrade every few years.  This plan didn't work out so will with windows XP, however, because XP wasn't mediocre.

Aged Windows XP costs 5x more to manage than Windows 7
As XP's life wanes, Microsoft talks dollars to get businesses to ditch 11-year-old OS
Gregg Keizer
May 25, 2012 (Computerworld)

Microsoft yesterday added ammunition to its increasingly aggressive battle to get users off the nearly-11-year-old Windows XP by citing a company-sponsored report that claims annual support costs for the older OS are more than five times that of Windows 7.

According to IDC, an amazing 42% of the Windows "commercial" installed base, or anything other than consumers' home machines, was Window XP, making Microsoft's job of moving everyone off the old OS by its April 2014 retirement nearly impossible.