Killing Windows XP Wastes Billions

Microsoft is already made up it's mind about Windows XP. It will cease providing support on April 8th, 2014. Despite the shutdown, there are still computers, millions of computers, still use XP because "it works" despite security concerns.
I always thought Microsoft was in the business of making money. If you are a shareholder, go to the next meeting and ask the bigwigs why Microsoft is leaving what can amount to billions of dollars on the table regarding Microsoft XP.

Microsoft is too dumb to realize that it can effortlessly monetize Windows XP using the beloved software subscription/rental scheme the company keeps discussing, but apparently has nothing but trouble trying to implement.

I am going to describe a multi-billion dollar idea that Microsoft must consider. 

There are still approximately 500 million XP users—an estimated 29 percent of the computers in the world. Many do not want to upgrade to anything new. They are happy campers.

Source: http://www.pcmag.com/article2/0,2817,2454844,00.asp

John ask why not exploit this by Supporting windows XP by just one buck per month. 500,000,000 million windows XP computers, pay one dollar per month = This will lead to 6 billion dollars, more than enough to improve XP for at least 5 years.

But the problem is that Microsoft can't upgrade multiple OSes forever. They have get people to update to Windows 7 and Windows 8 as well. Also, they do offer extended support for business and corporations starting at $200 a year (increasing every year.). But for Microsoft their minds are already made up.