One's entire equity portfolio should be sized based on the assumption
that the market might drop 90% and stay there for 5 years.

Under that assumption the size of my equity portfolio should be zero. I and my wife are both retired, and while we do have other income, the difference between a financially worry free retirement and a more budgeted retirement rests in our investments. The implications of an economic environment where equity prices would fall by 90% and remain there for five years are of such dire consequences that I would see cash – even considering its inevitable losses in purchasing power – as being the only reasonable choice.

I am willing to risk Buffett’s 50% warning, but not your 90%.