If you can't take stock volatility then you diversify more into bonds and stick to your allocation ratio. It's really that simple. Nobody can time the market. We will never get advance notice of a stock market crash.
Mentally, I (believe, untested since I had effectively no net worth during the financial crash) I can take the volatility of a 100% stock allocation, as long as I DCA'ed into my position over years, and especially if I DCA into it during market crashes.
The problem in this case is were I to suddenly inherit a lump sum, I really don't want to waste much of it in bonds, but I don't want to kick myself if the black swan cometh. Maybe increasing bond allocation and then DCA'ing the bond position into stocks over time might alleviate that somewhat, but I just don't have any positive feelings whatsoever for bonds at this point, and especially with a long time horizon where stocks wipe the floor with them. Bonds... as an alternative to market timing/DCA'ing and then reallocating over time might be a somewhat suitable alternative, or in conjunction with it...
Part of it for me is anchoring, and the value of your investment dollar. I'm barely at 100k/year and a 250k stash. If I was starting from 0, a 250k injection of money would be equivalent to ~5 years or more of saving. So avoiding having a year or two of that evaporate, not due to random volatility, but poor timing would be mentally, and economically devastating. Alternatively, anchoring to the 100k/year salary, pumping ~100k/year into the market on top of whatever savings rate you can manage works out mentally for me, even if after 2 1/2 years the market goes boom, I did what I could to invest properly. Adding a bond allocation to that and then moving away from it would probably be a prudent third step to the mental gymnastics. With a 250k stash starting out, the lump sum probably wouldn't be invested all at once, but the time period for the DCA would probably be shorter, say, <2 years.
Like I said, the math and Vanguard's own research indicates one thing, but avoiding getting mentally devastated and putting as much in front of that damage as possible is worth it, even if as an investor you might be comfortable with a 100% equity allocation, a lump sum falling into your lap that far outstrips a year of savings and/or your entire networth is something you treat a little bit differently, or at least, I can't imagine not treating it differently.