In the absence of individual strong shocks (such as the collapse of the largest banks, military conflicts, etc.), the bubble in the American stock market under current conditions may not burst for years or even not burst at all.
The reason is exclusively in low rates all over the world – investors will not take money from the American stock market, they simply have no alternatives. To sell something, you need to understand what you will buy in return. And there is just nothing to buy: yields are close to zero in all financial markets of the world, and some exceptions among developing countries, which are still capable of providing adequate returns, are low-capacity and low-liquid in order to shelter world capitals, a lot of money simply cannot go to them discreetly, without disturbing the prices.
That is why funds and other large institutions will continue to sit in the S & P500 (at least in dividend stocks), knowing full well that there is nothing to fear, in the absence of a choice, no one will run from here, but, on the contrary, with the slightest drawdowns they will only buy more. In this situation, it is probably not worth expecting some kind of collapse a la 2008.
It is possible that monetarists have finally invented a cure for stock shocks, albeit at the cost of turning the world economy into a global swamp.