Let’s assume that Trump falls 50 or more short of a majority of delegates when the primaries end, and Cruz is the second-place finisher.
Under those circumstances, I think there would be a strong case that the delegates should reject the plurality winner in favor of someone who got fewer votes. That case sweeps in his lack of any demonstrated commitment to conservative principles, his encouragement of political violence, his thoughtless illiberalism (e.g., his professed desire to have government crack down on press criticism), his unfamiliarity with a broad range of issues, and the mounting evidence that his nomination would be an electoral catastrophe endangering Republican congressional majorities.
What’s the case that the delegates should also reject the second-place finisher in this scenario? Near as I can tell, it’s a) that a lot of Republican officials dislike Cruz and b) that they have a hunch he would lose the election. Neither of those things seem like a good reason for nominating someone who got fewer votes, or no votes, in the primaries.