I've come to the conclusion that you will never really have a %100 secure OS, not even if you never-ever connect to the net what so ever. Why is this ?!? Because the pretty much all OS's have gotten rather complex, with lots of features to make a users life easier and/or more fun. Due to this complexity I feel that there will always be somebody out there that is going to look for exploits and vulnerabilities.
I bet if somebody dug into the old Amiga DOS v1.xx~v3.9 you could also find some issues like this. But what's the point in that there is a very finite amount of users that still have working, let alone use their Amiga's

Why not target an OS with the greatest amount of users so that when some new exploit is found that it can affect the greatest possible number of users ?!?