The idea that Trump was the only Republican who would have been able to crack the BIG BAD BLUE WALL is pretty amusing. I don't see someone like Kasich (or even Rubio or Cruz, for that matter) losing Wisconsin either. His victory map probably would have resembled Ron Johnson's victory in the Senate race (with Kasich doing a lot better in the suburbs but maybe slightly worse in the traditionally Democratic rural areas).
People really underestimate how many Republicans and Independents only reluctantly voted for Trump because of the Supreme Court and issues like abortion and gun rights and because they despise Clinton.
Cruz wouldn't have won MI, PA, and WI. He is too conservative for those states in a General Election. Yes Kasich could have carried MI, PA, and WI because he is a good fit in a General Election in those states.