I'm not sure you understand how rule 40 would play out in practice. Let's say that we get to the convention and Trump is the only one to get a majority of the delegates in 8 states but does not have a majority of delegates, and let's assume that they don't change the rule.
On the first ballot, Trump's name would be the only one placed into nomination. But unless he could persuade the uncommitted delegates to vote for him, he still would not win on the first ballot, because 1237 delegates is still needed.
Now we go to the second ballot, when most of the delegates are no longer bound. Cruz could persuade enough delegates in a few states where is already close to Trump (Louisiana, Kentucky, NC, etc) to switch sides to give him a majority of delegates in more than 8 states, so his name is now placed into nomination as well. And remember, just because the delegates were bound to vote for Trump on the first ballot doesn't mean that the delegate is actually a Trump supporter. Unless it is in a state where the candidates submit a slate of delegates, the delegates are usually long-time party activists, selected by the precinct, county and state party conventions - in other words, most of them are "establishment" Republicans...
The likelihood is that on the first ballot, if Trump is the only nominee, he will get the bound delegates and a fair portion of the unbound delegates and a fair portion of those bound to candidates that dropped out.
In other words, Trump will probably win on the first ballot, even if he doesn’t have enough bound delegates.
What’s more, it looks like Trump will win California and NY. Trump is going to go to the convention with enough bound delegates.