Well, the rule has been in place since 1856, so it kind of is carved in stone. If they didn't change the rule for Able Lincoln, for Ike, for Teddy Roosevelt, why would you think they would change it for Trump?
Nothing is carved in stone. As for why the party would adjust, it would do so to preserve the existing power structure. If a popular candidate threatens the power structure, there is good reason to block that candidate's take-over. Lincoln, Eisenhower and Teddy Roosevelt were acceptable candidates. Trump is not an acceptable candidate.
I don't think they would block Trump that way, I think there are less obvious ways to protect the power structure and cause Trump to fail, and by fail, I mean either fail to get the office, or fail to implement his policies once in office.