Can you even buy those anymore?

The GP is right: most new thermostats don't take power from the 24VAC line. That surprised me when my heat wouldn't come on one morning because the battery was too weak to pull in the relay for more than a few seconds. That's what I get for ignoring the "low battery" warning! All my previous electronic thermostats only used the battery as a backup.

In any case, are you really saying that using a toxic metal (mercury), or an imprecise bimetallic strip is really an improvement over a simple $10 electronic thermostat?

I guess when I think of a "dumb" thermostat I think non-electronic. None of the 5 places I've lived in as an adult has had an electronic thermostat.

I should have been more specific by stating the difference between a dumb digital thermostat as I was describing, and a truly analog thermostat like the Honeywell T87.

A dumb digital thermostat is just a thermocouple and a relay, which you could rig together with very little EE knowledge and a weekend with an Arduino.