It IS stupid to have battery powered thermostats. Here with temperatures far below 0°C, no heat for many hours can cause plumbing/water pipes to freeze after the home cools down. People have rural properties, leave on a vacation etc. and they are not always attended.
Berkshire Hathaway, instead of making a reasonably reliable Duracell product, have an armada of lawyers to defend their shit leaking batteries against the damages they cause. Duracell are the least reliable battery in the business.
google Nest used to sorta work without 24VAC power, but no longer. Nest does not operate without a "C" wire, the lithium battery cannot power it for the duration the demand triac/relay is on, it seems to be only for the WiFi chirps.
Because this screws consumers with 2-3 wire thermostat installations, they offer a "C wire adapter module" that uses all lines to parasite steal enough to make up a pseudo C wire.