I'm doubt we'll see Rule 40B stick around anyways. It's only been in effect one primary prior to this, and is obviously an unfair rule designed to tamper with the process by invalidating fairly won delegates. And of course it also blocks the establishment from putting forth a compromise candidate.
I've heard that Cruz was far more prepared for a floor fight before Trump even began to think about it, and that his celebrated ground game has stayed around in states after voting to try to influence the delegate selection to get pro-Cruz people in the state delegations. Whereas, I've hear that Trump was so confident of reaching 1237, that until recently, he just packed up and moved on after each state.
Rule 40B requires not just a win in a state but that you get 50% of the delegates for that state. What four states remaining do you think Cruz will win a majority of the delegates from ? Especially with Kasick still in the race. There are only MT & SD left in the winner-take-all type that Cruz could win and without WTA he is unlikely to get a majority of delegates.
I do think Trump will get over 1237, but there are definitely ways he could fail to get enough delegates even if he wins every remaining contest. With 3 candidates, it could go 40/35/25 in every state, Trump wins them all but has to split delegates except in the remaining 5 WTA contests, and Cruz never gets another majority of delegates anywhere.