Yes, the “time in the radiator” deal isn’t a thing. As you say, that’s an old wives tale and something that the laws of thermodynamics don’t bear out. Right with you there.
BUT, that’s not the whole story behind a thermostat. It’s more than just time to reach operating temperature, which becomes evident in cars that don’t have proper cooling at temperature without a thermostat.
Think for a second where the thermostat is. That restriction is backing water up in the block, not the radiator. The radiator has a huge advantage in dissipating heat, it has a large surface area as well as moving air AND coolant. The engine does not, only the coolant moves and the surface area isn’t optimized solely for heat transfer. The thermostat restriction allows more heat to transfer into the coolant in the block. Just think about how long it takes to warm your car up and compare it to how quickly it cools down when you’re moving at speed.
Running without a thermostat is bad, both because it takes longer to warm the engine up, which means it spends more time at lower than ideal operating temperatures which causes engine wear. And it also creates a less efficient cooling system once the engine is at temperature, because the engine isn’t transferring heat to the coolant efficiently. The factory could have made the thermostat 6” wide if it wanted/needed to, it is a restriction for a reason.
