Sorry, I meant top end of the motor - cylinders and head - not the upper limit of coolant temperature. You're right, that once the thermostat opens fully, it may as well not exist, apart from being a flow restrictor even when open. If the engine generates more heat than the radiator can dissipate, it will overheat regardless of what thermostat is in there.
Engine operating temperature (as measured at the coolant*) is an equilibrium point where the heat dissipation rate of the radiator matches the heat generation rate of the motor. On a properly-designed system, the equilibrium point is well below what the thermostat is set for and the engine ideally wants, so to bring up the temperature to optimum (however that's determined), a thermostat prevents the radiator from cooling if the coolant is below the thermostat's setting. Unless the motor puts out more heat than the radiator can dump - which it normally should not, even on a Delica - a lower-temperature thermostat will maintain lower coolant temperatures than a higher-temp one, and anything the coolant touches (such as the cylinder head) will also run cooler under normal conditions.
*To go deeper down this rabbit hole, operating temperature should be considered the average between hot coolant going to the radiator and colder coolant coming back.
OE temp gauges aren't hard data, but FWIW my 'stat-less Delica runs between the lowest mark and the lower normal mark most of the time. It took a hillclimb at 30C to get it ~1/4 of the way up the gauge. As far as the cooling system goes (disregarding heatsoak and components that the coolant doesn't touch), it's a proper system whose cooling capacity is greater than its heating capacity.
Some approximate numbers based on 4D55 data with an actual temp gauge:
No tstat, steady 100kph, ambient temps 20C, the motor puts out enough heat and the radiator dumps enough that equilibrium is 45C
No tstat, full power at 100kph, ambient 20C, equilibrium is 60C
No tstat, steady 100kph, ambient 35C, equilibrium is 55C
No tstat, full power on a dynamometer with no radiator fan, ambient 45C, equilibrium is 180C (kaboom!)
With tstat, steady 100kph, ambient temps 20C, equilibrium is 95C with the thermostat 25% open
With tstat, full power at 100kph, ambient 20C, equilibrium is 95C with the thermostat 75% open
With tstat, full power at 100kph, ambient 35C, equilibrium is 95C with the thermostat 90% open
With tstat, full power on a dyno with no fan, ambient 45C, equilibrium is still 180C and the thermostat is wide open
