Windows Defender has been shown time and time again in comparatives to be not much better than nothing. Its only plus point was it's low resource requirement.
https://www.av-comparatives.org/about-us/
I may be wrong, but I can't find any info as to their sponsors other than universities and volunteer researchers.
And I'm not sure how that addresses what I said about Defender being borderline useless, because they come up short across most tests from what I remember.
Unless it has drastically improved in the last year.
At the time (2010ish), they even had a few reports where they showed that pretty much all the AV products were quite bad. Either they failed to detect a large portion of real malware, and/or they were a huge drain on the system's resources.
Those aren't mutually exclusive... ;-) Some things never change, I guess.
As a Windows admin who uses Unix-ish systems exclusively at home, I am a little clueless sometimes. When using AV software on our computers, I can at least point to that and say I did what I could (be reasonably expected to do), but whenever I read what people who really know about IT security have to say about the AV industry, I get the feeling it's all just a bunch of charlatans and snake oil peddlers.
But the latter is a little difficult to explain to my boss without sounding like a tinfoil-hat-wearing lunatic.
romanovcode implied that because AV tests are frequently sponsored by AV companies, that would somehow negate Defender's consistently poor results (which would make no sense, as it has poor results pretty much universally across different tests).