Here's why:
They're thinking: If we endorse Trump, it will forever tarnish the GOP and alienate the Republican Party from most American voters; maybe it would be better to let Hillary win the Presidency and then try to regroup for the next election.
However: If Hillary wins, the Republican Party will lose all its power forever. The Democrats will pack the courts and polls and destroy the GOP. The GOPe's best bet is to persuade Trump to accept an establishment Vice President and go for broke. They can maintain their internal Party structure, and, if they win the Presidency, they can nullify the attempts of the Democrat Party to establish a permanent Democrat majority in the U.S. electorate by, for example, enforcing immigration rules and having conservative judges appointed.
If, on the other hand, Trump establishes a third party, the Republican Party and its Establishment will never recover their power.
They are also thinking: Let's go with Cruz. He's the lesser of two evils; we might be able to control him; and we might be able to snatch the nomination from him at the last minute.
However: Cruz will not be able to establish a third party; Trump might.
The best hope for America would be a Trump/Cruz ticket. This will hard to pull together but is not impossible, and the battle with the Democrats will be a hard one to fight, but if we win, we will win BIG, and if we lose, it will mean that we lost America years ago.
The only other good possibility is a Cruz nomination for President, but this would require some serious effort to prevent Trump from sabotaging it by establishing a third party. It might be possible, considering Trump's love for America and awareness of what another Democrat Presidency would do to America, but there would have to be a very wise appeal to him. This also would be difficult but not impossible.
The Republican Establishment had better do some extremely serious thinking.
They might not have the brain power required. Both Cruz and Trump do have the brain power, and the Republican Establishment had better listen to them very, very carefully and make the wisest deal they can.
The fate of America--and the world--is at stake.
I think there is zero evidence that your supposition: If we endorse Trump, it will forever tarnish the GOP and alienate the Republican Party from most American voters is true.
Actually the opposite is true. Trump is a populist and his ascendancy is a sign that the GOPe Establishment grip has loosened if not failed completely.
You are over analyzing things.
What a novel idea, to bad it comes from a lowly voter.