That might help if the exploit is in the third-party browser you are using (Firefox or Chrome) but no one can post a patch for Windows XP itself other than Microsoft.
I am not an expert MS coder, or even a professional one, so you may be right. However, I do know that quite a few rootkits and other malware have been discovered over the years that affected XP and sometimes its predecessors, and been publicly exposed and then fixed by some quite sharp 'nix coders. Often without MS even admitting they existed.
It does not take only a third-party application for an exploit to occur, sometimes it is simply a malware-infected executable of some sort. Even if that 'third-party application' is one that is very often necessary for several types of data -such as .pdf files for instance. It may not be part of the "official" MS operating systems, but most people need some kind of pdf reader at one time or another. Until you convince enough people that it is not a viable standard, you will still need to support that type of file and others as well so you need to pay attention to more than simply the MS base to keep a system clean.