There is little reason to estimate that the S&P500 return will be other than the long-term average.

There is a plethora of posts with data suggesting that this assumption is not to be taken at face value, at least not for the expected return for the 10 year time horizon. I can appreciate that you might discount those posts, and maybe conclude that there is always someone making the argument, and that the long term average is as good a guess as any. I can appreciate that since its a risk that we are all free to own. And frankly returns for those who took the risk have turned out well for some time.

But a counterview with plenty of reasons and data behind the logic of the position has been proposed. The sum total of which has been a lot more than "little reason" to we will be on the underside of the long term average rather than the upperside of the long term average over the next ten years. Any reasoning to think we are in for a period of achieving the long term average rather than substantial underperformance?