If the other 16 had pared themselves down for the sake of the party, they could have stopped Trump in the primaries instead of splitting the vote.
Which ones?
If we take out the poorest-showing ones, and distribute them around to the best non-Trump candidates, it still wouldn't be enough Plus, there's a decent chance they would've gone to Trump in roughly the same proportions that they went to the remaining candidates.
It would have taken Kasich to drop out and either of Cruz or Rubio. Only then would you have enough votes in enough key primaries to make a difference. A 1 on 1 against Trump.
And that just wasn't going to happen.