I would suggest the old units had a standing pilot and the thermo coupling was the safety that cut the gas to the pilot if the flame went out. A sail switch in the water supply line could open the gas valve supplying the burner.
I believe the reason they did not catch on was cost and servicing cost along with long time for pay back. Most would move before recouping the cost.
When schools use water resource heat pumps they use a closed loop cooling tower and boiler to maintain 80 degree condenser water. When you can provide a stable temp water to the condensers of the heat pumps you can get both cooling and heat year round. One side of a building can be cooling the other heating depending on control
