So... something new gets introduced that completely invalidates everything that happened before - but that's fine, because it's new and new things can never be illogical for the world just has to be transformed spontaniously so that the new content fits in to it?
Well, I disagree. There once was an entire Uchiha clan, and only a handfull of ppl ever awakened mangekyou. Only a few ppl of those learned to use Susanoo. And only those who gained the eternal mangekyou were able to use the perfect susanoo - btw, not too long ago the only person we knew if who could use it was madara, who at that time was considered god-like. Now Kakashi, who previously ran out of chakra frequently using his Sharingan just a few times in a fight, cn use it too, because why the **** not right?
Now I don't really care for how you try to call that, but I'm gonna call it asspulled fan-service for all the little girls who are like "omg I wanna see Kakashi with two sharingan and perfect susanoo, wouldn't that be soo awesome???? :333", because that's all it is.