It was disgustingly OP. Yes, they could have nerfed it, but he was already dominating the laning phase. He had so much damage that even once the passive was completely removed and replaced with something that gave 0 damage, his laning phase was still the best (except maybe Caitlyn).
Ultimately, they could have lowered his base damage or base scaling and increased his scaling and scaling scaling, but I imagine they felt like it wasn't worth the effort. I would have liked to have seen it been kept, because I hate snowball mechanics like the passive he has now.