The argument stated is faulty. I am no Romney fan, but the fact is he got the independents by a good margin. The argument that he would never get second chance from independents is simply not true. Obama got the base turned out in a way no one thought possible.
You can take it to the bank that, if the Pubbie rebellion dumps Trump and replaces him with Romney, 13 million who voted for Trump in the primaries are NOT going to vote for the establishment Republican replacement, be it Romney or Ryan or Cruz or Kasich or jBush or Mickey Mouse.
Trump won the nomination in the primaries by the rules established. Changing the rules to deseat him is certainly unfair.
Dump Trump and the Pubbies will lose the Senate and likely lose the House by 2018. And that will give Clinton a clean sweep to get any legislation she wants passed. Likely that will neuter the Second Amendment and the First Amendment. And look for upwards of 6 new SCJustices in the next 8 years.