Consider this scenario. There is only one operating system in the world that everybody uses. Now it’s going to be very easy for the attackers to write one exploit that runs on every single machine on earth!
On the contrary, if every single machine had its own operating system, then an attacker must write malware for every specific user.
The point from this argument is that heterogeneity of platforms makes it statistically harder on the attacker to write a malware that spreads well. The problem is that most of the personal computers on earth run Microsoft software. Recently, servers also are migrating to Microsoft. This fact makes the decision pretty easy for the attacker when he is choosing the platform under which his malware are going to work.
It’s very healthy for the whole internet to have some sort of balance between operating systems on both client and server sides to make the job harder for the black hats.
