There's no guarantee that such a "unity" candidate could have emerged, unless it was agreed on before Trump was looking like the eventual winner he turned out to be. And even if such a candidate *did* emerge, would enough people have voted for him? Carson and, perhaps to a lesser extent, Fiorina were drawing their support from people who turned to Trump once they withdrew. The only other potential opponent able to go the distance was Ted Cruz, who was even more hated by the GOP leadership (and everyone else). And that left a bunch of candidates that simply didn't have the appeal to extend to enough voters to challenge Trump. Jeb Bush, Chris Christie, even Marco Rubio (whose time may come again, perhaps). And no-one else who could have ran wanted to.
Trump's victory was well-earned, and well-deserved, and difficult to "stop". Put simply, enough people felt he was a choice worth making. The GOP party was broken already, and that is the void that he filled.