I'm disappointed but not surprised by this kind of sniping. There are many things that you could argue are optional add-ons to an operating system, but security is not one of them. Poor security has plagued Microsoft for a long time, and the buy-it-as-optional-equipment model is one of the causes. It's as if the car alarm makers of the world wanted a law that new cars had to be sold without any sort of locks or security systems. It doesn't make any sense.
Even if Microsoft built MSE into Windows, it doesn't mean that third-party security software is doomed. For an example, take a look at disk defragmentation software. A defragger has always been built into Windows, but other companies sell defraggers that offer performance and features not in the built-in software. Yet the built-in version is good enough for a lot of people, and it's definitely better than never defragging the drive.
If not for Microsoft's fear of the big security companies, I think that's where we'd be with MSE today. Basic antivirus and antispyware should be built into Windows. Enterprises will no doubt want more features than MSE offers, but MSE is a decent, free, and low-overhead security package for consumers that is a heck of a lot better than no security at all.