Stagnation is likewise not a terminal illness. And even though oil is our most important type of energy is it far from the only one.

A peak now can be an A shaped peak, a slow downturn, or a plateau that will regain strength. people, companies, states, markets, they don't even know when the peak happens, and only in hindsight is it possible to know when the stagnation that lead to collapse came. What might be seen as a stagnation by it's contemporaries might just be seen as a normal, war induced fluctuation in the near future.

I get you point, but things aren't that linear. Curves goes up and down, and what you describe might happen so slow that it it barely recognisable, or filled with so many periods of ups and downs that it is hard to detect the long, downward spiral. It is extremely hard, close to impossible to use such a a metric as an indicator as you suggest. It is too volatile in it's nature.

And to add, the mix of energy types are also changing, as it always has. Oil is the most important, but not the only. Even if it is in part riddled with the same issues would a "peak energy" metric be vastly better if we want to try to interpret the sate of the global economy.