Because bloatware antivirus software has enough annoying notifications already. Loosening it again to having their stuff shown all the time makes new devices even more annoying for the general user, who doesn’t reinstall a clean Windows version on first use. I don’t see unlimited notifications for AV software as useful at all. It’s the stuff that every Windows user I know, who isn’t tech-savvy, hates about Windows.
And second, I never heard of an antivirus not working after an update, but of course it can happen and I won’t deny that it did since I just don’t know, but that should well be solved by giving them final builds earlier (even though I don’t get why they can’t just use the insider builds to test it) and securing the system with Defender automatically after a certain time period without an AV software won’t hurt anybody, but help the general population that needs a system that updates and secures itself since the users won’t do it by themselves.
Imo this is not about people that know something about their system, but ones like my parents, who need a system that always works and is secure. And as long as Windows 10S is in the state it is now it is important to do other measures to live up to the standard population that clicks notifications away and doesn’t update anything.