I'm not sure I buy the theory of not enough time to dump heat. There is certainly enough anecdotal evidence of overheating with no thermostat that is corrected by installing a thermostat or restrictor plate though. I wonder what the real mechanism is.
It's not a theory. Heat rejection may take very little time, but it takes a finite and measureable time. If you don't allow that time, heat rejection (at least to some level) just don't happen, and you can't get heat out of the system.
The engine rejects combustion heat into the coolant, but the coolant has a limited capacity to accept it. The coolant then rejects heat into ambient air (which has a theoretically unlimited capacity to accept it,) but this takes time. If the time isn't allowed, the coolant won't be able to reject as much heat into the ambient air. If heat rejection from the coolant doesn't occur, then heat rejection
into the coolant
can not occur - it's elementary thermodynamics. The ambient air can accept rejected heat until it reaches the temperature of whatever is trying to reject heat into it - since ambient air is almost never 215*F, that's not a problem.
Just like on a hot day - you can't cool down, because the ambient air runs somewhere between your skin temperature (~70*F) and your core temperature (~98.6*F.) On a very hot day (ambient temperature 98.6+*F) you won't be able to cool down - because your body can't reject heat into a medium with an ambient temperature that is at or above the medium trying to be cooled. Sweat allows for this somewhat - since sweat is a collection of moisture on your skin that is allowed to evapourate, the water accepts heat from your body, which is then rejected into the atmosphere (until you cross into a location of ~80% RH or higher - and the air is unable to accept the water vapour.)
When that happens, you jump in the pool. You jump into a denser medium (more readily accepting of rejected heat) with an ambient temperature well below your core temperature (most summer swimming pools I've been in run 60-72*F - well below core temperature, and often below skin temperature.) Or, you rub something cold on yourself, usually somewhere that blood runs close to the surface (whether you know it or not. Forehead and insides of wrists are popular for this reason.) Heat may then be rejected by your body into whatever you're using - often a bit of ice, or a cold drink in a metal can (which provides additional heat sinking anyhow.)
Energy will always flow from higher potential to lower - until the lower potential is saturated, and
if sufficient time is allowed for the energy transfer to take place. The more energy, the more time it takes. Batteries run until they can't generate a potential exceeding the requirements of the circuit they are feeding. This takes time. Coolant rejects heat until the ambient air won't accept the heat anymore. This also takes time.
Some energy (heat, for instance) takes longer than others (electricity, say.) Electrons move at a sizeable fraction of the speed of light - typically 70% or more, depending on the wire - while heat moves much more slowly...
EDIT - This has a lot to do with why racing engines run "restrictor plates" in place of the thermostat - they are essentially a thermostat without the poppet valve in the centre. Reason? Flow volume restriction, and heat rejection.
