The GOP has already blown itself up. It doesn't matter wetherDonald gets the nom or not.
Trump is a symptom of the diseased state of the GOP. Too many years of ignoring the voters, ignoring the base, and caving to the democrats at every turn.
Trump will lose the WH and probably the senate and maybe the house, but Cruz most likely loses to Hillary too. So the question becomes what scenario is more likely to set the party up for 2020. If I'm a party insider, I don't like Trump, but I can also see the gop has to make a large shift to attract middle class voters. Trump is poison with the Latinos, but the party can lose this election and get past the immigration issue with something they can sell to the base as at least securing the border.