Technology

the wisest move is to hoard cash

I disagree, market timing fails more than it succeeds. The better bet IMO is to diversify your investments. When one bubble pops, you'll have assets in other sectors/regions/etc that aren't in a bubble and get the investment dollars from people fleeing the bubble.

Warren Buffett hoarding cash doesn't mean you should hoard cash. He's hoarding cash because he's a sophisticated investor with a long track record of being able to find good deals, and he sometimes buys entire companies outright, and having a large cash balance makes that a lot easier. He also frequently funnels that money into stock buybacks instead of leaving it in cash. He doesn't know if the market will crash next year on in a decade, because as you said, the market can remain irrational.

Do what Warren Buffett says, not what he does: buy and hold a broad market index fund (he recommends the S&P 500).

That's what I'm doing. I'm rebalancing my investments more regularly because I do expect this temporary run-up to drop, but I'm unwilling to try to time the market. I have a target US/international ratio, and I'm making sure that's correct (my US portion has grown faster than international). I have also decided to pull the trigger on a small-cap value tilt after watching some good videos by Ben Felix, so I've been completing that transition as well. I intend to keep this portfolio for >10 years (probably through retirement, but we'll see what happens when there's new research).