I have no doubt there will be a correction at some point, but that wasn't the OP question.
Vanguard was merely forecasting that US large caps, which have outperformed for a decade, would underperform vs. other categories over the next, largely due to current valuations. Not crash, just grow less quickly.
And I was asking whether anyone was going to reassess the 'buy and hold a global tracker' strategy which has worked so well for so long, given US/large cap concentration in even a global fund.
The answer might be "all forecasts are rubbish, chasing the market is pointless, I'm sticking with my tracker".
Or it might be "My tracker is no longer diversified enough, I will be looking for pockets of value elsewhere".
Or many other variations I'm sure. All valid, all can be considered positively.