@davidrmoran That I have a harder time believing as inflation tends to be outside of corporate and government control at some points. There is a belief though that low rates will be here for a while, yes, but "permanent?" like you said, with quotes only.
I think though this discussion is rather abstract so let's put some concrete details beneath it as to what is really underlying the permanently elevated U.S. large stock valuation scenario:
1. Does one believe that Apple, Amazon, Google, Microsoft, Facebook, and Netflix will dominate the world until the end of time or will either governments or competitors, perhaps foreign competitors such as say Samsung, Ten Cent or Alibaba eventually begin to erode their marketshare? They are today's bellwethers. If they slip, you can almost be certain the U.S. market crashes. It seems even under the current ultra-rightwing administration there has been some pushback against their monopolistic tendencies.
2. Do low interest rates last forever? Last time they inched up, market cracked?
3. Do low corporate taxes last forever?
4. Does U.S. labor remain powerless and declining forever, having no bargaining power or pricing power on wages?
5. Do commodity prices stay low forever?
6. Corrolary to 4 and 5: Does inflation stay low forever?
7. Do merger-friendly, monopolistic, anti-competitive anti-labor right-wing policies remain in place forever?
8. Is there some accounting scandal at any of the aforementioned bellwethers about to break?
9. Is a war--a real war not the virtual one already being waged--about to occur?
10. Do the nationalistic trends we've seen throughout the globe ultimately mature into full-blown fascism in the U.S. or in major trading partners?
11. Will climate change and the policies necessary to keep it from becoming even a greater ecological catastrophe than it already is restrict business growth?
Any of the above could throw the "permanently elevated" stats askew.