I dont think Kakashi has some Senju dna.. you dont see him being strained.
I agree. Let's remember, Kakashi doesn't have THAT MUCH chakra. And the deterioration of the eyes is how often you use it in a certain ammount of time. Kinda like Naruto's Rasenshuriken. If he used it by himself (not in Sage Mode), he could use it once or twice and the damage would heal in a few weeks. If he uses it again before his arm has recovered enough, his chakra network is damaged beyond repair. Similar concept with the Mangekyo Sharingan.
Itachi's eyesight was damaged little to none by the time he fought Sasuke because he didn't use it very often, and it was over the course of ten years. Sasuke spammed it like a fool and destroyed his own eyes in a few weeks.
The reason that the Mangekyo Sharingan deteriorates your eyesight is because if the chakra strain in the ocular nerve. When the eyes are transplanted and foreign chakra/eye tissue kicks in, the strain has no effect. It just uses up chakra.
Obito didn't transpland his eyes, but it's INSIDE the half of his body that's composed of Sinju DNA. This prevents deterioration because of two main things:
A) The emminse ammount of chakra allows him to use it more, and such and advanced flow allows him to use it withought strain
B) No foreign eyes, but it's in an artificial part of his body, wich implies that the Senju DNA attatched itself to his eye during recovery and prevented the ocular strain that an ordionary Uchiha eye socket/nerve would have experienced.
