The quote I recall is "Why keep trying to run up the score when you've won the game." I see this as asking when you have enough (however you choose to define that amount), why maintain an AA that is subject to significant volatility.
In my own situation I view that as going from a 60/40 AA prior to retirement to a 40/50/10 AA. Considerably more conservative, provides a nice cash "bucket" in the event of a prolonged downturn, yet has sufficient equities to hedge against the unlikely return of high inflation.
Moving to 100% fixed income investments is too risky for my blood.