I think history favors projects, companies, and individuals that concentrate one something as opposed to big entities that try to be all things to everyone. This is how small companies are able to dislodge larger ones. Focus is important in anything. The argument is far from flawed, as limited counter-examples are hard to find.
Tons of projects with a single focus have had countless security problems, some so bad the entire project was completely scrapped.
So the entire argument is based on a flawed premise.