That only works so long as it doesn't collapse like a house of cards due to being overvalued beyond all justifiable reason.
Asserting that the US economy as a whole is worth 50% more than it was 3 years ago is delusional.
Asserting that the US economy as a whole is worth 50% more than it was 3 years ago is delusional.
As an example, 3 years ago Starlink was little more than an FCC filing. Now it is poised to create a brand new market conceivably worth as much as Apple.
If you think Intel is overvalued compared to AMD, you can short Intel, buy AMD, and make a bundle of money if you are right, so there is a mechanism to keep relative prices in line. If you think the 2020 market is overvalued compared to the 2012 stock market though, you can't move money back in time. Valuation doesn't tell you if/when a crash is gonna happen, it just gives a vague sense that returns will be lower over the next 10-20 years. A small positive return beats nothing, so a rational actor that knows the market is overvalued will stay invested. Hence, there isn't really any reason to believe market valuations are so rational.
I tend to think the market is overvalued right now, but I'm buying stocks anyway, because I'm saving money and don't want to stuff it under my mattress.