>> My theory is that if you do not open the door to a none thermostat oven, the temps inside
>> will remain rather stable and not bob up and down.
>> ...
>> At any given wattage you will eventually reach an equilibrium.
Again, Yes, it could be done, and Yes, you could bake things in an oven made this way.
But you're still missing the big picture. That equilibrium equation includes losses due to opening the door (necessary to put food inside to cook, or to check cooking progress), the thermal mass of the cold food and cookware you put in, the temperature, humidity, drafts, etc of the external environment in your kitchen, the materials and insulation of your oven, the insulating value and thermal mass of surrounding floor and cabinetry, whether your nearby refrigerator is running, whether the lights are on, whether the sun is shining through the window, what season it is, whether the oven light is on, etc.
It is very easy to design a simple temperature-regulated control circuit to hold an oven at a specified temperature +/- a tolerance, regardless of the outside conditions. It is near impossible outside of a carefully controlled lab environment to reach that same temperature at an equivalent level of temperature stability by equilibrium alone.
So what do you do?
If you set the heater wattage to reach that theoretical equilibrium point at a sane cooking temperature, you have to deal with an oven that for practical purposes never reaches temp. Because with cold food put in, the time it takes to recover is on par with the total time you cook for.
If you set the heater wattage to be higher to account for that, to bring the temp up faster and keep cook times in check, then your equilibrium point is a much higher temperature and leaving it on will burn your food if left alone.
If you increase the thermal mass of the cooking appliance to improve temp stability (cast iron stove walls, etc), you increase the warm-up time considerably, worsening the problems above. If you reduce the thermal mass of the cooking appliance to improve warm-up-to-equilibrium time, you worsen the impacts of opening the door or inserting food.
See the problems?
