The only problem with continuing to support any OS which is no longer maintained is that, in many cases, we are implicitly supporting users on platforms with significant and well known exploits. This is especially true with Windows XP, which has numerous, well documented and easy to exploit vulnerabilities.
While I would agree that the decision to use a vulnerable OS is up to the individual user, in practice, many users don't understand the risks and consequences. To make it worse, exploited systems can also be a threat (or inconvenience) to other users (for example, by being the source of botnets or DDoS attacks).
There is an argument that it would be more responsible to not support any OS once it is no longer maintained and receiving patches for security vulnerabilities to discourage continued use of vulnerable systems and encourage users to update to a current and more secure OS (which of course could be GNU Linux!). This may also make maintenance of Emacs easier as it would reduce the need for exceptions and work arounds for older systems which don';t support more modern OS practices.
Tim