What's wrong with avoiding Microsoft anti-virus, too? Or os he giving this advice only because most people can't disable Microsoft's antivirus anymore, anyway?
Because my guess is that Defender is reasonably implemented and does the basics right, and therefore is significantly better than nothing for the average user. That guess is based on anecdotal observations of the quality of developers and practices at Microsoft vs what we've learned about A/V vendors via bug disclosures.
