Do you actually deal with end users?

'Open Source really is more secure, because ANYBODY can look at it to identify faults'

Unfortunately, so can hackers looking for exploits. With the code available to them it makes finding problems so much easier. You can bash MS as much as you want but at least they have an idea about the average user by trying to make things like security updates as easy as possible.

If Linux or some other OS where to suddenly make it big (70%+ of desktops) how many problems do you think this would cause globally?

Think about it, a large proportion of the computer using population want everything to happen automatically. They can't even find a document they created if they save it to the wrong folder or make sure their AV is up to date, believe me, I get loads of work dealing with little non-issues like this. If a vulnerability is found in Linux how many of them do you think will check every now and again to see if they are still secure? How many will have the ability to replace the kernel with the newer patched version? Very very few. most would never even think about it leaving the internet open to flooding by botnets taking advantage of a vulnerabilty across millions of PC's. You think it is bad now, it could be so much worse.

And before anybody says it, no I am not a MS fanboy, I just deal with end users every day and see first hand just how many people haven't a clue and they also have no interest in learning about the systems they are using. They just want to switch the computer on and it works, without them ever having to do anything or learn anything about it.