Edit: Changed title to be more accurate.
Also here is the summary from Wikipedia on what Post-scarcity means:
Post-scarcity is a theoretical economic situation in which most goods can be produced in great abundance with minimal human labor needed, so that they become available to all very cheaply or even freely. Post-scarcity does not mean that scarcity has been eliminated for all goods and services but that all people can easily have their basic survival needs met along with some significant proportion of their desires for goods and services. Writers on the topic often emphasize that some commodities will remain scarce in a post-scarcity society.
Everything is unsustainable. So far, nothing has been observed to last forever.
Fascism tends to collapse a lot faster than capitalism though, because of its inherent drive to rework the world into something new. Fascism cannot abide non-fascism existing, and contains the implicit imperative to invade and impose fascism.
Just want to add this is not unique to fascism. It is an emergent property of all systems that seek to reshape the world.