Indeed, if we are going to argue for an exponential curve we'll all be retiring soon.
Compounding returns are in fact exponential, and it is amazing, and yes, you can retire on them (though not too soon). This is why most personal finance books highlight the "magic" of compounding returns in the first two chapters and the importance of starting early.
OP, is it your contention that the S&P500 should never go above 1500? That the economic gods become displeased when the S&P500 has the audacity to flirt with 1500? That the number 1500 strikes fear into the heart of investors? That corporations in the S&P500 are only capable of earning enough money to support a valuation of 1500?
If the second crash happened when the S&P500 hit 1400 would you be less worried? If it crashed when the S&P500 hit 1600 would you be more worried? Is the very tenuous "pattern" that two crashed happened at 1500 somehow important or telling? Does the fact that the S&P500 has risen to 1700 mean the curse is broken?
I could go on, but you get the drift. Nothing "wild" here.
To your last comment, the world GDP continues to grow as humans become more and more productive, and industrial technology reaches more and more people. S&P500 companies have an increasing number of affluent customers to sell their products and services to. You really don't need anything crazy or disruptive to support earnings growth.
I think its reasonable to assume we won't have another 5 years of increasing stock prices. So, how to play the next five years, or do you bother making any strategic changes based on this?
So you're basing this opinion almost entirely on the fact that the S&P500 fell when it hit 1500 twice? Can you really argue that the world is in a different place that 1982 or 1990? Every era has its reasons for bullishness and bearishness. Sometimes bearish sentiment is the best reason to get long. Sometimes, when it looks like clear sailing it's the most dangerous time to invest. I just don't see any arguments that corporations can't continue to grow in this environment. Is it not entirely possible that the corporations can continue to grow, even as the fed tapers over the next couple years?