1000 Trump delegates and 700 Cruz delegates are going to be cool with voting for Mitt Romney? I think they might shoot up the building instead.
The Trump delegates will definitely just keep voting for Trump. But the rest of the delegates will presumably go with whatever candidate their guy endorses.
It is definitely also possible that Cruz/Rubio/Kasich just refuse to compromise and keep insisting the other guys should switch to them and the primary goes on forever, but I assume at some point somebody will come up with a lever that moves them. I have no idea what that lever is.