In a perfect world, something like this would be possible because everybody would get along and be able to work toward a single goal. In reality though there are tons of conflicts of interest and personality in addition to the standard technical/development conflicts about things like licensing, project goals/focus, acceptable limitations, etc..
Practically speaking, the open-source community is better off because of competing projects/OS's. Just look at the smaller subset of just the BSD operating systems for instance: OpenBSD, DragonflyBSD, NetBSD, and FreeBSD all have different goals and focuses, and each one develops and implements different types of technologies - often to replace some existing piece of software. Once that new tech has "proven" itself, it gradually spreads beyond the originating OS and gets imported into the other BSD's - for the betterment of all.
If there was just a single, monolithic "OS" it would most likely end up limiting creativity and reducing the effectiveness of that OS in the long-term.