Look for a supervisor, tell the story, make them aware that you are aware that since the delay is within their control, you have every right to be rerouted on another carrier since they have the next available flight.

If you still meet resistance, ask them if they rather pay the agreed interline fare by issuing a FIM or endorsing your ticket over to the other airline, rather than you buying a (more expensive) walk-up fare and claiming the expense back from them later on.

At this point, I have yet to meet further resistance - has always worked for me to actually get what I was due (=being put on the next available flight, which happened to be on another airline).