Where do you see evidence that Ravanna is too powerful, that the dharmic order of the cosmos is being upset? 


Ravanna is able to change the seasons. He gets too hot after hearing about Sita and plunges the world into nighttime, then into winter, then back to the sun, then into total darkness all in the same day. It confused the crap out of the worlds inhabitants and completely messed up the order of the cosmos. Ravanna also has kings in his court who will not put their hands down for fear they will be killed for not bing able to bow properly when the time comes. He too has gods under his control. It's too much power for one being to have and so Vishnu has come to restore order.