^^This, a well known fact.
The mask took all the impact of the Rasengan. It was just freaking durable.
This is not a good logic. If he didn't have a Sharingan, he would have trained his other ninja fields. We know he is capable of it, because he became jonin at the age of 13 even before obtaining a Sharingan. Also, he would not be able to use Chidori, but would still be able to use Rasengan, which in most cases can be a replacement for Chidori. (Even though it's weaker than Raikiri, that's true.)
He would of course be weaker than he is at the moment, but not by such a huge margin.
By the same logic you can say that Naruto would be fodder if it wasn't for Kyuubi or that Obito would be weak if it wasn't for Senju DNA (which presumably is what allows him to spam his Kamui).