I mean it's just science that the planet can only sustain X people at Y life style (you can swap out "life style" for "level of consumption"). Like there is hard rules of physics about quantity of water, minerals, et cetra. So you can either lower X, lower Y, or try to side step the problem entirely by going to other planets/mine asteroids/magic to increase the resource pool.
The sustainable level of consumption is dependent on population.
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)