lol talk about multiple forces, I remember in 1984 I thought I might solve the 3-body problem (n-body problem, more generally).
Yes, a big issue with economic or social prediction is that people are adaptable, both as individuals and as groups--in their behavior, attitudes, even morals. In my experience few economists are ever "right" about more than one thing, for more than a rather brief window of time, even "greats" like Friedman. Look at how Greenspan has been changing his tune.
To the immediate discussion, reasons for the current bull market--money moving in because return is favorable compared to fed-rate-suppressed gov't bonds, and "new" long-term money moving into index funds. Reasonable suggestions, for sure.
But I think they don't tell the whole story, and that there are other factors driving the equity markets, factors that I have talked about--algo's, international money, wealth preservation, and unfunded public liabilities. Yes everything is relative, but I don't see any of those changing any time soon. Unfunded public liabilities are always rising, and quickly. Wealth disparity, especially internationally, grew tremendously under Obama, and there is now a very, very large wealth preservation class who are secure. International capital flows have proved to have been very resilient in the face of efforts to curb them, and all indications are that their rate will increase over the coming year. Finally, it is incontrovertible that the use of algos and other automation shows no signs of abating, in fact quite the contrary.
The only real wrench is the fed rate, which has held very steady for a long time. Inflation, even by their own measure, is exceeding their target, suggesting a rate hike soon, but when and how much is anyone's guess--maybe a slight bump in March or April? We had a slight bump at the end of 2015, and the markets are now significantly higher without any significant increase in economic activity. 2 hikes in the past 10 years and 3 predicted for this year? The ones that are coming are already overdue, and in my mind, already somewhat priced into the market.
Yes TIPS are a well-worn wealth preservation device, but they come with a certain set of risks. Any wealth preserver hedging risk will want other vehicles with decoupled risk. On that point, look for EM equities to now take off, for that very reason--because there is more decoupling from US Treasury risks than there is for the US equity markets. As long as foreign jurisdictions make it relativity easy to invest, and maintain a rule of law sufficient to relatively fairly adjudicate disputes and have a modicum of political stability, they will feel the benefit. I have already bought some in anticipation of this effect.