The problem with your statement is that "stealing" unbound delegates in primaries is like stealing bases in a baseball game - it's part of the game. The delegates Cruz is "stealing" are the one that are not bound to vote for anyone and are free to choose. Trump is supposed to understand the (complicated and nonsensical, but still clearly written) rules of the primaries and (1) have delegates in every district, and (2) recruit unbound delegates to support him. If the author of The Art of the Deal cannot close the sale with even a few unbound delegates, he's not the person he claims to be.
I want Trump to be the nominee, but I want him to win under the rules as they are currently written. If Trump doesn't hire the experts he needs to understand our stupid primary system, what makes you think he will hire the experts he needs to understand the rare parts of diplomatic nonsense that actually matter, once he is in the White House? If Trump cannot learn the rules well enough to beat Cruz in a GOP primary, what makes you think he will pay any better attention in the general election when he faces a corrupt expert who specializes in genuine vote fraud, like Hillary?
You don't remain in the race just to stop the front runner from getting the number so you can try to win the nomination at the convention.
That is why what Cruz is doing is so unethical.
If they were neck and neck in the race that would be a different issue and a convention fight would be legitimate.