Microsoft should sell Windows XP because its newer operating systems should be so much better than Windows XP that its customers would be happy to buy the newer versions.
At the moment Microsoft 'forces' its customers to buy the latest versions, which creates a false perception about the real impact and value of that release.
The fact is that Windows XP is quite a nice operating system, still better/easier than some Linux distributions out there, and with really small hardware requirements (for 2013 technology). The other day I was asked to install Windows XP for a friend and apart from the tons of patches and reboots that were needed, the end result was quite a nice/fast machine (that did 99% of what that PC user wanted)
And that PC user was happy to pay for an updated version of XP (and office 2003) from Microsoft. Instead she had to spend tons of time on ebay to find it. Why doesn't Microsoft have a windows store that works on XP? That is a massive missed opportunity.
The other 'unquantified' but very real problem, is that the Windows development teams should be forced to complete with their previous versions and not be given its customers on a plate (or should that be 'under the gun'). The current model of forced upgrades only works when the new versions are 'orders of magnitude' better than the previous one (like ms-dos to Windows 95 or windows 95 to windows 2000). Once we reach the stage of 'incremental upgrades', the real situation should not be masked by 'forced updates' and situations where users HAVE to upgrade. In fact Apple is starting to have this problem, and the fact that they keep selling the previous versions of their phones at smaller prices, shows that they value the competition (and development model)
There is value in Simple solutions, and it should be Simple to go out and buy a 'Fully updated and patched' version of Windows XP.
Finally, since Microsoft is not willing to support Windows XP any more, it should (be forced to) release its source code under an Open License (ironically GPL would be the better one for Microsoft), so that others are able to continue to support and maintain it