This is the number one thing overlooked. People trying to mitigate volatility always do so assuming that they had that AA at the height of a market crash and not before. The risk of opportunity cost is far greater.
Which is why static portfolios are foolish, IMO. Your life changes, your goals change, your investments should change with them. You are correct in that there is a huge opportunity cost of someone with a 10-15 year accumulation period holding 30% bonds throughout. That literally makes zero sense, assuming the goal is FIRE ASAP. This person should be 100% equities for at least the first 75%.
However, as time goes on the same person may become sensitive to FIRE date, so decreasing portfolio volatility has advantages. After that, for the first five or so years of RE, this same person is at the height of sequence risk, so adding additional noncorrelators makes more sense. Later if this person gets to a 2-3% WR due to good returns or a windfall as discussed above (ie almost dividends only), then noncorrelators are worthless to goals and only serve as a drag to runaway wealth.
Why do we always assume a well educated mustachian is so stupid that they need to hold the exact portfolio mix from beginning of accumulation until death? It's not "market timing" if one adjusts portfolio to mitigate the risks associated with their phase of life or financial goals.
One other point, the assumption is not bonds or other non correlated assets remain static during a general market downturn. The point is they generally perform better than their average. Capital often flows to one or more of those other asset classes, that's one of the reasons the general equity market drops.