In order for that to actually be a money-making strategy for Apple, those third-party apps that address weaknesses in the OS would have to be sold through the Mac App Store so that Apple gets a cut. I've been a Mac user since before there was a Mac App Store, and I've never bought such a utility through the App Store. I have paid for several such apps over the years in ways that did not generate any direct revenue for Apple, and most of those apps likely could not be distributed through the App Store because of how they muck around with private APIs and other OS internals.

Those third-party apps do increase the overall appeal of Apple's platform, but suggesting that Apple might want to encourage that situation rather than improve their OS themselves sounds like a broken windows fallacy.