The actual rule is that the candidate must get a majority of delegates actually voting. Five hundred no-shows would mean 250 fewer votes needed to win. That’s why alternates are always present.
Since delegates are bound by state rules, they can’t be unbound by convention whim. Most state parties are going to remain jealous for their prerogatives regardless of the candidates.
What if the majority of Trump delegates (and alternates) fail to show up, while all the Cruz delegates show?