Lower oxygen would be anemic, while greater oxygen would be too flammable. Dead planets find equilibrium by geological circuits. Gases, such as carbon dioxide, dissolve in liquids and can precipitate out as solids. Only so much gas will dissolve before it reaches a natural saturation. Solids can release gases back into the atmosphere when heated and pressed by volcanic activity.
Thermodynamic entropy draws all chemical reactions down to their minimal energy level.
The furnace metaphor breaks down. But the Earth has the self of a thermostat. A spontaneous circuit, provided by the coevolutionary tangle of life, which guides the chemicals of the planet toward some elevated potential. Presumably if all life on Earth were extinguished, the Earth's atmosphere would fall back to a persistent equilibrium, and become as boringly predictable as Mars and Venus. But as long as the distributed hand of life dominates, it will keep the chemicals of Earth off key.