I think you and a couple other folks hit it directly. If the local agency (sheriff, fire chief, police chief, EMA chief) won't embrace interoperability, its a lost cause. I helped a small bit with the statewide fire/EMS mutual aid that came into Columbiana Co- interoperability was a big issue once units got to the area.
One of my biggest comebacks to people who refuse interoperability is this: it would not be tolerated if an officer was as inept with their firearm, a paramedic was as inept with their narcotics, or a firefighter was as inept with hoseline deployment, as they are with their radios. If the agency isn't going to demand proficiency of it's users, turn in that $5,000-$6,000 radio, exchange it for a $1,500 radio with smaller capability, and use the money for something else. But, don't get on the news or at council meetings and lie to us all and say you can talk to your neighboring agency with your fancy interoperable radio when we know you couldn't if your life depended on it. (sorry, rant off)