MS has stopped supporting XP, so there isn't any obsolescence to plan for. The only updates I have seen are those for net.framework and they haven't slowed anything down that I notice.
I have noticed that with each SP release for XP that the system restore function gets worse and worse until finally after SP3 you can only use it from within windows if you set a point for installing something and use it soon after you don't like the install. If you wait even a day or two, the restore will fail and you need to go into safe mode before it will work again. Its things like that which make me lean towards your assumption that either MS screws with the OS or at least fixes somethings (critical updates) at the expense of other functions. I still use XP although not so much online as within my network and for certain games. There is a lot to like about XP SP1 and a little of SP2 as well but if I'm going to have an offline gaming PC I'll use SP1 unless there are restrictions for things I want to install that require SP2 and I'll forgo SP3 altogether.
most modern applications require XP SP3 + Windows Installer (MSI) 4.5 so foregoing SP3 may not be the best option besides, all the XP security patches released from mid-July 2010 til April 2014 (and maybe beyond) require SP3
never experienced slowdowns with post SP3 patches in fact the only thing that can do that is .net s**tty framework and their garbage collector
Technically you can still get updates if you trick your PC into thinking it is POSReady2009 Embedded, they are compatible with XP fully as POSReady2009 is just XP with a few shell edits and changes. I recommend Xubuntu or Lubuntu if you have a low-end PC, however you can continue to use XP if you like.