If that happened it would be far worse than the great depression, that would have the S&P an non inflation adjusted return of 1.70% annually for 24 years.
Inflation adjusted the market at 2745 is the equivalent of year 2000 top of 1545 , ie a negative return for two and a half decades
I think you are answering more in a non zero chance fashion rather than what you actually believe. If you thought that would happen you'd be a fool to keep your money in the market. Obviously you can just pick any number above zero and say sure that can happen. But we are making actual decisions and allocations of capital and we have assumptions about the probability of future events when we do so.