@joenorm, excellent questions IMHO.
Yes, I wonder about these. A sociologist by nature, I wonder about them a lot. I see markets as just a part of society; societies as pieces of the global network of human culture.
Opinions to date:
Climate change - good example of a major risk for the global system. Is the global system guaranteed to overcome all risks? Surely not. Are the odds good? Not sure about the long term. In our lifetimes, probably a problem but not one that will kill most of us, or even 1% (I think). So markets will probably prosper. (Probably.)
More generally, I think the broad economic future for humans is a battle between our growing power to damage natural systems vs our growing power to recycle resources, or use existing resources more efficiently than before. I suspect there will be times when the damage part makes stock markets fall, but times when the efficiency part makes them rise. My guess is the rise aspect will prevail, but that's not a guaranteeable thing.
In the short term, mere decades at a time, it seems to be obvious that there will be swings but less obvious that the swings will be larger or smaller than the last century. It could be either. I suspect the past century is a good guide. For perspective, I think the recent Federal Reserve study "The Rate of Return of Everything" is awesome.
https://www.frbsf.org/economic-research/files/wp2017-25.pdfFor more recent periods only (roughly 1970 to present), portfoliocharts.com has great data summaries for multiple asset classes, including investment results for numerous sample portfolios that mix different assets. Diversification is a great strategy to protect against uncertainty, and can work well for returns as well as safety. Best of all, it has detailed calculations and data for multiple countries, though far from all countries.
https://portfoliocharts.com/Fwiw, one key result from portfoliocharts.com is that not all countries have seen as much stock advantage as the US. One idea that I consider is the possibility that in future, US results will be more like those of other countries. If so, less stock and more diversification might help.
This last idea may be wrong, of course. Maybe America's markets have idiosyncrasies that will last forever. Seems unlikely, but I have no way to know.
Final thought: To me it seems one possibility is that returns will be lower because of falling population growth. This could happen soon enough to affect most Mustachians, I think. But again, I don't know how to predict.