A glass can hold X amount of water. You add X plus 1 and water spills out. "Guess the glass couldn't sustain X water!" is what you are claiming?

Systems can only be sustained within certain parameters. There is no magical system that is always sustainable regardless of parameters. But it sounds like we are saying basically the same thing in any case.

Consumption can go down and more population can be supported. Or consumption can continue at current levels but population will have to go down. (and that level it would have to go down might be truly horrific)