In the case of a Prius I wouldn't change the thermostat unless the car was overheating. Preventative maintenance is a good thing but in this case I would pass . The thermostat is hard to get to and requires a few hours of labor to replace. As for your statement "Thermostats can't last forever" no not forever but I know of a few vehicles with over 300000 mile that have never had theres changed . The dealer is probably being honest here . Most dealership look for ways to ding you . I would be thankful they didn't say they changed it when they hadn't and then charged you for it . Hope this helps