I suspect Cruz would still be down just because he has too many negatives, but he'd be running a highly disciplined and very tactical campaign and he wouldn't be down by much.  One false step by Clinton and he'd surge.  Kasich is an actual adult and would be coasting to the White House right now.  Rubio could have easily beaten Clinton too, and if he'd won the nomination the Republican Party would be burying its image as a haven for angry old white people instead of proving itself to be nothing but.  Rubio would also have attracted younger voters instead of driving them away.

Trump will be for the Republicans what McGovern was for the Democrats: the product of a grassroots insurgency who pleases the extremists but leaves normal voters convinced the entire party is nuts.  It took the Dems twenty years to rebuild their presidential prospects from that one (without Watergate they'd have never gotten Carter in).  Republicans are in for a long drought.  They did it to themselves.