If people wouldnt login as admin in windows, and actually took care of it like many of us linux and mac users do, I doubt that there would be the number of problems plaguing windows as there is. Not to say that windows is quality, its far from that.
When i think about linux having the user base that windows does, alot of "normal" average users, who dont know how to use the computer, who dont care about it, all I see is exactly what we have now. Perhaps it wont be the kind of worms and virii we have now, but something just as bad will come out of it.
Of course Linux is much more stable than windows, and isnt written nearly as poorly, but that does not prevent it from attack. When linux has 90% desktop control, you think people will focus on writting exploits for windows? They will focus on linux. And there will always be retards who login in as root, or donot update their systems, who will allow these exploits to fuck their systems up.