Rule 40 does not favor Trump since Cruz is working with the GOPe, trying to install his own delegates as shadow Trump delegates (Arizona for example). Were Cruz an honest candidate willing to win or lose with the voters I would agree with the premise.
Trump needs to make an alliance with a candidate that will help him get the necessary 1237 and win in the general. Kasich seems a likely ally in that regard. He helps with women; he helps with Ohio; he helps with Congress.
My understanding is that candidates cannot keep their delegates after the first ballot and that they aren’t permitted to trade them at all.
Many, many people are continuing to insist that Cruz is hated by the establishment, and that they will attempt to replace him with an establishment guy.
Too many are saying that if Trump doesn’t win it on the first ballot, then he won’t win it at all.
So, the only protection these two have is rule 40.