PE is an impaired metric. It's not a constant - even company to company. Accounting standards change, a new CFO introduces some new policy, and boom: you can't even untangle a single company's real non-mumbo-jumbo earnings per share without a CPA and a dozen hours of reading reports.

What we do know about PE, generally, is that companies want to try to keep it as low as possible, and that at the end of bull markets, that's especially true. This issue gets really crazy when a company's c-suite is heavily focused on boosting share price at the same time.

But let's assume that today's PE ratio is more or less accurate.

From an investor's standpoint (not a trader or speculator) that means when we buy a company at 22 times earnings, we're saying we either don't mind waiting 22 years to capture the value of today's earnings (on paper, not even talking about dividends, which likely paint an even worse picture) OR we're saying we expect some incredible growth in the near term.

The S&P500 is a not a growth index. There's evidence suggesting companies only enter the S&P500 when they're about done growing. I don't have the data, but the index represents some of the largest companies in existence. Most of them can't get much bigger...

Could valuations get even crazier? Of course. But someone looking to buy and hold stocks can get wiped out for decades if they buy at the wrong time. If you're at or near retirement age, you don't have decades to recover after a decade of paltry or negative returns.

Right now the bull market in stocks is ancient. Valuations are high. Earnings are likely to be constrained. Dumb money is getting very speculative. Interest rates don't have much room on the low side. These aren't good signs for healthy long term returns.