Now this is all ASSUMPTION since it was never really clarified in the manga.
Why is Kakashi not going blind? Easy. Cuz his other eye isn't a Sharingan.
Even if his Sharingan eye went haywire from overuse, he can still see fine with his other regular eye. Now I know what you all are thinking: you can't see as well with only one eye. So I take you back to Kakashi's appearance. He has his Sharingan eye covered most of the time to keep it deactivated, meaning he is used to only seeing from his regular eye.
So there you have it: Even if Kakashi's MS went blind from overuse, Kakashi wouldn't BE blind cuz he has a normal eye.