How hot is hot?

I can see where I was not clear. Obviously it sets the minimum because it doesn't open and allow coolant to the radiator.

The point I was trying to make is that if there is sufficient cooling capacity, the temp will still be regulated by the thermostat because it will not be wide-open and no longer controlling the temp.

A mechanical vehicle thermostat is NOT a simple on-off switch either completely closed or completely open (unlike the wall stat for HVAC in buildings). I've actually heated them in pans of coolant and observed the gradual movement of the valve.

It opens more as it gets hotter, thus increasing coolant flow through the radiator (which of course then lowers the system temp and it closes down partially). Eventually an equilibrium is reached which should be somewhere in the operating range of the thermostat. The time constant of the loop is several seconds.

So, if the thermostat is wide-open it has lost control of the loop, and then (as you point out) the maximum temp is indeed limited by the rest of the cooling system.