[QuasiSocial]

Apart from trying to kill each other, they also sap a lot of resources and fight over viruses. For example, if they both detect malware and one quarantines it, the the other will annoy you endlessly.

I agree that sometimes they try to kill each other. I also agree that they use a lot of resources.
I don't think I have ever seen them fight over what was found but I guess that would be possible if the antivirus expanded what it considered a threat.

While my surfing practices don't expose me to much Malwarebytes is most often what I see blocking on my system.

I run Kaspersky antivirus because I won't run without an antivirus.

A well written virus or rootkit will be invisible and only do what it needs to do quietly in the background so that you don't go looking for it.