Come on. The reason this software exists is because programming at the hardware level is insane. The OS exists as a platform for the masses. Could there be a single OS, absolutely. Can you say more insanity though?

Options, barring there aren't too many, are true freedom. That being said, the developers do things as standards, and then they add something to make themselves unique. No matter how far in the future we go, If I were developing a commodity type item such as an OS, I would still want to differentiate it from the restk; otherwise, relevance of the OS would be a 'do-do' waiting to happen.