Someone like Kasuch, Rubio or Cruz may have been a "better choice" than Trump, but would have been murdered by Hillary.
Actually, the polls showed that any of those 3, Kasich especially, would've easily beaten Clinton because of her high negatives.
At this stage you arent saying that you trust the polls, right?
The polls became tilted in Killary's favor. Even taking into account bias for her, they predicted that many democrats would vote for Kasich or Rubio.
Trump easily handle each of those three. She would have too. With the media and the establishment on side, even smooth clean Marco Rubio would have been fitted up. Poor old Ted would have been stitched up worst of all.
There's a big difference between Republican primaries and the general election, it's a totally different dynamic.
In primaries, you have to appeal to the party base. Trump did this better than anyone else. In general elections, you have to appeal to the moderates, centrists and independents. Both candidates sucked at doing this, Trump was seen as a loose cannon and Hillary as corrupt. 3rd parties got more votes this cycle than in any election since Ross Perot in '92.
Both Trump and Hillary had big negatives and most people who voted for them were simply voting against the candidate they hated the most. You introduce a candidate without big negatives and they get all the independent votes plus many who will cross party lines. Kasich would've wiped the floor with Hillary in a general election.