I've had one experience involving UA IROPs in Montreal. Got rebooked in a matter of minutes onto DL. UA agent wanted to downgrade me to economy but in the end got my ticketed cabin.

What the US needs is a passenger protection bill that imposes service recovery like we have in Canada except that I STRONGLY DISAGREE with paying compensation ( except in vary rare cases) due to delays.

Airlines ( especially in cases within their control) should be required to reroute on any airline's next available flight in the same cabin as on your ticket. I can never understand why some agents try to downgrade passengers when rebooking on flights that have premium availability.

Right now it seems to depend on each individual agent as to how the situation will be handled. A law allowing the DOT to massively fine both the airline and the employee ( or airline management if they don't empower their employees)who refused to reroute on another airline ( if proven that seats were available) would prevent these situations.