Ah, clarification: you're OK with using a proprietary (and intelligently proportional) thermostat, as long as there's a backup dry contact option if the factory thermostat fails and is irreplaceable.
Yes, that’s true; I’m ok to use a proprietary thermostat if I think it will save me money in 2 years max. (I’m using Sensi wifi thermostats that will become non-programmable if they stop supporting their phone app. It’s ok, as they were around $40 each after incentives and I’ll save most of that just this heating season. Its interface to the boiler is just power, common, and call-for-heat.)
At least in a hydronic (air-to-water or gas-fired) application, an intelligent stat is not mandatory to get decent efficiency (from variable control). You still have building heat transfer rate and outdoor temp sensing available inside the unit even with just a contact closure.
