All I'm asking is for a simple policy that states something such as "if there is space on another carrier and the new carrier accepts DL tickets then we will accommodate you on another carrier".

All cases I have experienced have involved a single broken aircraft as the reason for the IRROPS. Not some huge weather problem like some of you seem to focus on. AA and UA left with empty seats to ORD. My grandmother had to wait until the next morning and take a connection in ATL. I can't think of any good reason in that scenario to refuse a carrier transfer.