So what are you saying exactly? Itachi's 3T was severely deteriorated from continuous MS use, did that change the fact that he had it activated for most of the time he was on-panel? Nope. Same goes for kakashi, his 3T looks fine yet he's already told us that he can't see well w/ the eye.
Point being the 3T sharingan doesn't cure bad eyesight, never stated or implied.
The Sharingan allows to read movements perfectly clear along with several other abilities as well.