I believe he actually did eventually notice his vision getting worse, hence why he kept his left eye covered. Not to mention that time he encountered "Itachi" when trying to retrive Gaara from Akatsuki, and asking him "how bad his eye sight is."
Yet when we see through his point of view during his attempt to kamui the gedou mazou his vision in mangekyo appears normal and without deterioration. Compare to itachi's vision when fighting sasuke.
_____ Obito is Tobirama's great grandson. The proof is in the pudding.