Obitos MS isn't ur average sharingan my friend, besides there's barely any strain on the eyes when they use Kamui unlike Amaterasu.tsukuyomi and susano'o which leaves the users covering their eye because it hurts.
I'm fairly sure its amaterasu, susano'o and tsukiyomi that hurt the vision. I don't think Kamui or Kotoamatsuki (have no idea if that's spelled right) will blind you, I think techniques specific to a users MS are fine
Kakashi didn't rely on Kamui until this very battle. He always used it as a last resort, as opposed to Sasuke. That's why he was able to keep his eyesight good for so long. As we can see, his eye lately started bleeding, proving that he's still susceptible to MS' sight deterioration.
Obito, on the other hand, has Senju DNA, which more than likely is the reason he can spam Kamui and not care.