When you look at most reources you will find that the richest 10% make up about half of the global resource consumption. That is not just true for energy, but also for things like food due to meat. At the same time there is a minimal base for resources and we are going to adapt to new circumstances. Especially with resource constraints not being a total cut, but a slower process, this is bound to happen.

It becomes even more striking, when you look at how resources are disributed globally. Brazil for example is in a decent state. Able to provied enough food for its population and has a good bit of renewables. Obviously poverty and crime is common today, but it is certainly a workable position. Europe as a whole might be able to just cut down and survive for a bit. Nearly 29% of primary energy of the EU come from low carbon sources and the EU is food independent, with massive overconsumption of energy and food. So cut backs and some adaption might make it work.

Poin is that it is that a 90% death rate is very likely massivly overstated. Some regions will be in trouble, but not everybody will be.