Why Equities: Bonds ratio is 60:40? But why?

I would guess that Equities swing up and down. So fixed income swings other way around and 60:40 sort of event it out. But this is a very simplified approach.

I sort of expect that if equity (broad market index) cash, they will eventually come back up. So during the time between equity swing down and come back up, I need a way to withdraw funds without selling equities. It is also expected that if equities crash, feds eventually (1 year from the crash) will come to the rescue and try to lower interest rates. My strategy here would look something like this:

• keep one year of withdrawal in Money Market. - I will use it to cover first year while market gyrates. I also will sell some equity to realize income at 400% poverty level.

• keep 20-year Tbills to cover next 3-4 years of withdrawals.

- In case if market crash, I will wait until feds lower interest rates, the TBill principal will go up and I will sell them to cover years 2-5 of the equity market downturn.

• Everything else I keep in equity (like S&P, International, a little bit in emerging markets).

Hence the ratio is not 60:40, but more like 5 years of withdrawals in fixed income and the rest in equity.