That argument is indeed the core of it. Every other antivirus needs to be in your face constantly so you think you're getting value out of the purchase. They

That argument is indeed the core of it. Every other antivirus needs to be in your face constantly so you think you're getting value out of the purchase. They all suffer from the "this rock keeps tigers away" problem, that their enemy shows up too infrequently to justify their own existence. So they need to noisily invent enemies. MSE's existence makes a ton of sense when you realize the reason for its creation. It was developed to reduce Microsoft's own support workload, as a significant proportion of Windows support tickets are caused by both malware and belligerent antimalware. MSE justifies its existence by reducing OS support costs rather than by annoying users into paying for antivirus. So Microsoft's incentive is simply to have users use MSE, which is nicely aligned with delivering a clean quiet positive user experience.