Energy is not only a constraint, it is the real driving force in economic growth. Its price or share of GDP is another matter. Classical economic s
is just what it sounds, classical. It means it has been developed at a time when economic growth was very slow and it models basically a static economy. The world has experienced sustained, long term rapid economic growth only about 50 - 60 years (this is at the level of 3 - 6% a year).

But here we speak about energy, not only oil. The '70s is an interesting period: the oil consumption really decreased considerably. But the total energy consumption did not. There was probably a small drop at the time of world energy crisis but otherwise constant growth. A sizable part of oil consumption could be substituted by natural gas, nuclear energy and coal. But now? This would be much more difficult.