Demystified - Do Espresso Shots End?

http://rowanxeus791.unblog.fr/2021/03/15/exists-such-a-thing-as-a-dead-espresso-shot/

Crema is the layer of foam that normally sits on top of an espresso that has simply been pulled. The typical rule in coffee making is that when the crema vanishes from an espresso, the shot is dead. Hang on: why