Maybe he already has it.
Think how often he uses his MS, Sasuke used his only a few times before he lost his sight.
Kakashi can see fine, still.
And have you ever seen his eye not activated?
He's not going to go getting a Perfect Susano'o or anything but his Sharingan does seem to last forever.
It puts a strain on him because he's not an Uchiha, not because it can't be eternal.
Eternal MS still puts strain on the user it just doesn't blind them with continuous use.