They know President Cruz will bring The Art of No Deal when they try to do business as usual. By Trump's own admission (bragging, he's actually proud of it) they've done much business with him before. They want the political marketplace to remain open for business and they're OK with Trump as the proprietor.
What you and others don't seem to comprehend is that the "deals" are only bad because Republicans keep making bad deals. They give away the store to the left. What us Trump supporters are hearing is "Not on my watch. I know a bad deal when I see it. I'm going to only make good deals for the country."
But he talks in vague little sound bites and his campaign is pretty much sloganeering and personality.
If you listen to what he is saying, he is not a conservative. He may be a big nationalist, but he is a compromiser. That's how he works. He lays out a position on the far edge of an issue (like immigration - Deport everybody - No Muslims until we figure this thing out) and then when he goes in for negotiations, he meets in the middle (OK we will only deport the bad ones and the good ones can stay).
He is not principled. Never has been. Never will be.
If you are looking for a first class compromiser. Donald Trump is your guy. Vote for him.
If you are looking for a principled conservative, then you are going to have to look elsewhere.