It's definitely not an argument for a portfolio that's 90% S&P 500 and 10% short term treasuries unless the time frame is more like 25 years instead of 7 years.

I have always considered that advice to be fully suitable only for those that are so wealthy (like Mr Buffett's widow) that rate of return is of no real concern.

It also has a big hidden problem...when does the person spend cash, versus when does the person sell stock to top up the cash? If the person in question is capable of valuing the market and making a sound decision on that subject, they don't need advice on the cash/stock allocation. If they don't have that skill, the advice is a bit incomplete. Presumably the cash cushion is there for those instances that it's better to use the cash than to be selling equities--but what's the test for that? Does the heir know how to answer that question?

For example, if the idea is that no such decision is needed, merely to keep that allocation at all times, then in effect it means that every withdrawal entails selling stock to raise cash to keep it at 10%, which is roughly the equivalent of never using the cash, which raises the question of what it's there for.

Even the simplest advice in the world is not that simple.

Simpler and safer for someone a little less plutocratic might be a TIPS ladder yielding inflation + 2-3% these days, in some mix 0%-100% with annuities. Way fewer decisions. At a certain age, longevity risk and planning is a much bigger deal than long run real rate of return which an equity-heavy portfolio does probably maximize.

Jim