If I was going to construct a rule, I'd make it so that delegates are bound as long as the candidate to whom they are committed is one of the top two vote getters on a particular ballot. Or maybe limit it to the top four on the second ballot, top 3 on the third, and top 2 on the 4th, so it couldn't go more than 4 ballots.
Why should someone who bothers to go through the process of becoming a delegate be bound forever to a candidate who can’t even be bothered to try to get his own people selected as delegates?
Delegates do other things at the convention besides vote for POTUS. These delegates should be people who bother to show up for the party, not just people assigned by a candidate who can get people to vote every 4 years.