It would be an A without this ending.
<spoiler>The movie puts us in a position where we know that Ellie actually went through a wormhole and met aliens (assuming reliable narration). Then the public has no proof of this fact, but she's urging them to believe her nonetheless. So this is a situation where 1. There is some truth. 2. There is no proof of this truth. 3. The only way to access this truth is through faith. This is a very religious and non-scientific message! Don't urge anyone to believe in some stuff you say if you have no proof! What happened to "make me a liar" when she discovers the first signal? I ain't even mad at her for this moment of weakness, because she's visibly emotionally overwhelmed, and the morale might as well be that scientists are bad at keeping their emotions in touch, but clearly this is just a "lol sometimes you need faith to be true" kind of morale. Imagine for a second the same ending, except we don't see the wormhole scene from her point of view, we just see the ball falling through the machine from the external point of view.</spoiler>