Randissimo,
You are talking yourself down a very deep hole, will you ever reach the bottom?
You have already stated that no machine is truly safe, no antivirus or antimalware app is perfect, and any machine can be compromised.
Ergo any machine whether XP or Win8.1 could be part of a botnet until the problem is noticed/found/removed.
Look at the Worm/Virus section of this forum to see the well protected machines that fall down.
So again your reasoning and argument against using XP falls on its face.
Using Deep Freeze , the entire system is reverted to a clean state on each and every reboot, the Windows system cannot be infected and any problem is removed.
Better still is to use measures to reduce the risks of landing at bad sites. My systems have been clean and safe since 2006 .....