1. Obito mastered the use of his Sharingan where he can use Kamui without it transforming to MS.
or 2. Since he has one Sharingan instead of both. It would take a lot longer for him to go blind since both eyes are not in sync.
The True Power of The Mangekyou Sharingan comes with a hefty price. You have to remember that Obito has never awaken his eyes true power. I won't say Senju DNA like a moron because Kakashi has Mangekyou Sharingan as well. He experienced slight eye deterioration since he questioned Itachi about his eyes.
EDIT:
Manga never mentioned Kakashi eye sight deteriorate. I don't know if he read the Uchiha tablet or not.