Everybody who gets a smart thermostat needs to keep their old one in a box for the day the server gets EOL or the device and/or network gets hacked. When that happens, you aren't going to be able to go to HomeDepot and buy a new dumb thermostat.
Dumb thermostats will always exist. You could also preemptively wire one up right now in series with your smart thermostat to act as a safeguard against extreme settings.
