We agree return temps should be lower, but to determine how critical this is you need to out numbers on that. In steady state operation at maximum load a difference in return temperatures of 15f is ~1% efficiency. But steady state at maximum load is an extreme situation the average return temperature is well below that theoretical maximum thus an even wider difference is needed for a seasonal difference of 1%.
Redundancy is critical in areas that get really cold. That may eventually mean turning to hydrogen, but a backup gas furnace for a well insulated home really isn’t a major CO2 contributor. More relevant to the discussion it further reduces the impact of minor changes in efficiency or comfort.
