"Unless I totally misunderstood my readings, AVs and malware detection applications are niche services. " I think this depends on the particular software, the intended message and context of the conversation. Just as an example (not intended as proselytizing), I use what I think of as Norton Anti Virus. But it's really the Norton Internet Security system. It's a suite of many, many programs and routines that handle a large number of functions. Website reputation, antivirus-malware-spyware-phishing blocking and removal, firewall, temp file cleanup, safety of files downloaded from websites or emails, email scanning, and on and on. Other than the occasional message that pops up, it's invisible. AND, it schedules scans on the fly for only when the PC is not in use. It never interrupts or delays other activity. I never touch it, never schedule anything, and almost never see anything. I think all the best in class products (there are maybe 4 or 5 best choices) are similar to what I described, comprehensive "systems". They're well worth the modest annual cost.

Avast should automatically disable windows defender when you install it. And should automatically re-enable it when you uninstall Avast, but that doesn't always work.. sometimes, and its a good idea.. that you visit avast support to see if they have a special uninstall cleanup tool.

I have never had a virus until I started prepping for W10. When I start downloading my software, I got something that locked up my toolbar, safesearch I think and wajam. I did a system restore, then a friend told me it was still in restore and told me how to get rid of it there. Later I picked up a virus with no name, it just had gen in the name with other stuff. It froze my Avast and my PC. I waited a few minutes and I was able to use IE. Downloaded Adware cleaner and that took care of the second one.

Both Android and the Apple operating systems for phones and tablets aren't 100% safe (nothing in life is) but they're both sitting on customized versions of Linux (which has pretty tight security features). I've never heard of anyone having a problem with either types of devices but I'm sure it happens, however rarely. The best security is a careful user, no matter what device is involved.