We have countries today that treat women as second class citizens and still practice slavery and they can be considered "thriving" societies which are completely different from the Western world which can also be considered to be made up of "thriving" societies.

So if the requirement for a moral system to be "true" is for it to result in a thriving society we have a problem because we still have competing moral systems.

And even if we somehow whittled down to one global system, just because people believe in it doesn't make it true (do you need me to provide examples of nearly universal beliefs that were proven untrue later?)