Of course there’s the possibility that both Cruz and Trump could be “mathematically eliminated” from reaching 1237 before the convention. In that case, it will be up to the hundreds of “other” delegates (won by Rubio, Kasich, et al.) and unbound delegates to put one of the two over the top. That doesn’t mean Cruz or Trump “stealing” any delegates from the other guy. It’s a matter of all those non-Cruz/non-Trump delegates preferring one of the two over the other.
Neither Cruz or Trump will be the nominee