But the magic of friendship is
already controlled by a single being. How do you think the God Emperor Celestia obtained her position? By being a peaceful and just ruler? Hah!
It's only a matter of time before someone overthrows her, whether that be Luna (she'll have to find some friends) or the mane cast (they'll have to realise what is going on), or even someone else who gains control of the elements of harmony. One god pony has already been defeated, why not the other one?
Then once you've removed the old powers, there's nothing stopping the new owners of the elements of harmony from ruling the world for good, with their insane and devastating friendship powers, and they'll be the only ones left with that kind of power.