Is that really so hard to believe? Thor may be the strongest Avenger, but he's not exactly a brilliant mind. Carol combines Asgardian level power with actual tactical and strategic skills. Less powerful than him, sure, but moreso than all the others - and a little less power with a little more forethought could easily be the key. Thor himself could've actually won already had he followed that formula, but that's not what he does.
And anyway, I hardly think she'll rescue everyone and kill Thanos all by herself. She'll be important, but so will Thor, Cap, Tony and many others.
And anyway, I hardly think she'll rescue everyone and kill Thanos all by herself. She'll be important, but so will Thor, Cap, Tony and many others.