One reasonable answer, I think, comes from the Kelly-type analysis analysis that I cited a few years ago. I don't have the citation at hand, but the conclusion was that the optimal asset allocation depended on the difference between the expected return of stocks and the expected return of T-Bills, as follows:

expected difference between stocks and T-Bill returns --> optimal stock allocation

4 percentage points --> 100% stocks
3 percentage points --> 75% stocks
2 percentage points --> 50% stocks
1 percentage point --> 25% stocks
0 percentage points --> 0% stocks

Right now the expected 12-year nominal return of stocks is about -2%, per Hussman, and 3-month T-Bills are yielding 4%, so if this Kelly analysis is right and this stock forecast is right, then the optimal allocation to stocks is 0%. Historically stocks have returned 9% since 1871, and T-Bills have averaged about a 6% yield, so the optimal allocation to stocks over time has averaged close to 100%.