That's all well and good but there are viruses and worms and so on that can appear on your machine no fault of your own, it can just take a backdoor into Windows which is why M$ is endlessly doing security updates. An example I can think of that struck the world hard was Sasser, look it up. " I won't get the flu, I avoid busy public places and wash my hands all the time "
I'm wondering about the privacy issues with Windows Defender in Windows 10. For example, if you turn real-time protection on in defender, and if you open some file on your hard drive, now defender will definitely scan this file in real time. But will defender send messages to M$ about your behavior of opening this file and the file names, size, etc., regardless whether this file contains virus/spyware? This is much more important than resource-eating I think.