If the whole world focused and used just 1 OS for every system for a long enough time line, I think it would evolve fast enough to reach a point of perfection, where there are no security holes or flaws of any kind. I do believe that while programming has many ways of doing the same task, there is always an objectively best way to do it. Eventually the best way to do everything an OS needs to do would be found; it would be faster if there was only 1 OS to work with to reach that point.