I'm pretty sure Senju DNA is the reason for that. If it was able to reduce the cool-down period for Kotoamatsukami from 10 years to 1 day I don't see why it can't prolong the users eyesight. Not to mention Danzo had no control over it, and he only had his shoulder done.Yes, thats a big ass plot hole because Sasuke uses his MS a couple of times and he goes almost blind, in the other hand Obito is been using that S/T cheese since he was 14-15 now currently 29-31 years old he doesn't go blind...