If Teemo gets the nearsight effect some other parts of his kit will have to be changed. If he gets nearsight his cheese with his passive will be too toxic. To make him fair his Q damage will gave to be gutted so it becomes utility only. This will make AP Teemo a lot weaker so he will need other parts of his kit buffed to make his damage relevant.
Also he will not counter other AA-based champions anymore, and will have even harder time vs fighters with gap closers like jax and irelia. Nearsight is good on quinn and graves because they can disengage. Teemo would get popped and would easily die if he gets jumped on.