They don't expect you, because you can easily disable automatic downloading.
The entire update thing is very easy to configure. If you want to use the most used OS, then you are going to have to deal with updates. The most used OS (or any software) will be hacked to shit in comparison to its less used brethren. What is the point in writing an virus, finding an exploit in, or doing anything involving that type of work in regards to something that is less than 10% of the userbase (market share)? It is more worth it for hackers to fuck with Windows users, because they can fuck over more people than they could if they were wasting their time trying to fuck over the users of other less used OSes. The more popular something is, the more haters it gets.
Which leaves the obvious question, would Macs be as vulnerable and easy to hack as PCs, if the market share was a split 50-50? Or would one still have an advantage?
Macs run off UNIX. People have been able to hack and fuck up UNIX based OSes in the past. I don't see why Macs and any other OS platform would have a security advantage over the other. Anything can be reverse engineered. I don't see a 50/50 split happening. 50/30/20 or whatever. There could be 3 or 4 viable OS platforms for people to choose from, problem is that only one of them would have enough software programmed for it to get the most users.
You don't see people giving a shit about making games for Mac or Linux like they do for Windows, that is because the userbase for Windows is so much larger. If 50 percent of the people used Macs, then you might see a split that is closer to what is happening development wise with videogame consoles.
You seem to be lost in your own thoughts.