I'm not sure I agree.
If the brakes fail on an automobile due to poor design, the automobile manufacturer is held responsible and liable.
It follows that if Windows exposes customers to O/S Security Flaws, MS should be held responsible and liable also.
With the above concept in mind, it would logically behoove MS to design the Best Security Tool possible in order to protect itself against liability.
Just as 3rd parties compete and claim to have better browsers than MS IE, they can also can compete and claim they have better security tools than MS MSE and Defender.
The real problem is that with no oversight and/or regulation, MS has been allowed to write its' own market place rules and is accountable to no one except its' share holders. "Too Big to Regulate" and "Self Regulation" - I think we've heard those terms before for sure !!