What do you mean fiat doesn’t work on a finite planet? Current economic models certainly don’t work on a finite planet, but fiat was here before them and will be here after they are long gone.
what is a good way for the working class (90%+ of all humans) to save and succeed in this current environment?
There isn’t one. A big chunk of that class can do just fine and you probably already have good normative answers in that respect, but the current economic model is one that demands poverty. Even with all of the ridiculous developments in production we have, the available infrastructure even with the qualms we might have with it, and all the other things going for us that you might want to list, the closest that the current economic model has achieved to escaping its age-old need for having a sizeable portion of the able-bodied population unemployed is by slightly expanding that same portion and then having them sell themselves by the hour and minute in the Gig Economy. If you want that whole 90% of the population to all be able to do well, you need to change the system they are operating within.