Linux

I don't disagree with what you're saying necessarily, but I feel like you're giving Microsoft too much credit here. There are anti virus solutions on Linux. This isn't a thing they're doing because they see an unserved market and a new way to make money. This is a thing they're doing to get a foot in the door with orgs that are switching away from Microsoft products to Linux (like many EU orgs recently), so they can try and draw them back into a 365 subscription or whatever.

So, yes, some businesses may need an anti virus solution due to contractual stupid reasons, but that isn't what makes Defender on Linux "sensible" for anyone except for the Microsoft salesperson.