The future is not only unknown but also unknowable no matter who is making the prediction. Bear in mind that all predictions regardless of source are merely guesses. The market seems to take pleasure in sometimes humbling all predictors including its most intelligent and well prepared experts. Having said that, the following is my guess. I think the risk of a huge stock market decline that is sustained for more than 5 years is exceptionally low, so low that it should not the dominant force in choosing your asset allocation unless you are extremely rich and can thrive long term with the zero to negative inflation adjusted long term investment returns which is what "safe assets" currently offer.
There is no way to avoid risk in investing if you require significant positive real returns to meet your financial goals. The question is which risk bothers you more--an unlikely equity market collapse that may last for a decade or more (very unlikely), or running out of money when you're old and unemployable (more likely IMO). Each of us must set that balance at a level appropriate to their own situation. IMO don't look for a cookbook answer to tell you how to do it. The cookbook answer may turn out to be more flawed than yours. Know yourself and your financial circumstances well and do what makes sense.
There is no question that the stock market, bonds, real estate, etc., are very overvalued by historical standards. We are however living in an age that has never existed before--zero or negative real interest rates for more than a decade, incredibly massive QE and fiscal stimulus, and in spite of all that, sluggish growth for 14 years that many expect to improve substantially in the near term. That conjunction of events has never before occurred in US history or any other country's history as far as I know. Historical measures of valuation have been thrown out the door. The market is overvalued but it is hard to accurately define by how much. Investors are now expecting robust future profit growth near term as we emerge from Covid and the economy reopens,. Investors have already priced that expected future good news into current valuations.
Market expectations for future corporate profit growth are currently very optimistic, perhaps too much so. On the other hand, where else are you going to put your money? All US quality investment assets may be overpriced but that can last for decades. There is an incredible amount of money concentrated in the investing class in the US, all of it looking for a place to go that's not overvalued and yet has good future prospects. That's why there are few if any such stocks now. They were bought long ago. There are two backstops that work in equity investors favor no matter how high valuations get: the FED with unlimited financial resources, and the incredible amounts of money that have to be invested somewhere.
There is a very remote possibility that could cause a long term market train wreck--persistent, substantial, and increasing inflation over a long period of years. It appears that the odds against that are overwhelming as Powell stated and the overwhelming majority of investors believe. Strong secular forces (aging demographics, globalization, tech innovation increasing efficiency, massive debt worldwide, etc.,) are expected to cap inflation after a modest upsurge this year. All governments in the developed have been trying with every tool they have to create inflation for 14 years, Japan for 3 decades, and none have thus far have succeeded.
I believe portfolio moves to consider at this time include significant international equity exposure and modestly overweighting value stocks which relative to growth darlings look more sanely valued. Possibly a small allocation to GDX, a leveraged play on gold which to date I have not done. Value and INTL will benefit if the global economy starts growing robustly which is expected to happen. If you want real gains long term diversified stocks are IMO still the best instruments to use as they always have been. Just don't expect a replay of the incredibly great gains we've had over the last 4 decades. If the future merely rhymes with the past that will be enough.