You have to let go of the "in a limit down situation" thinking. Unless you are already retired and on fixed income, this mentality is limiting your returns.I just started doing that; earlier I had about 70% SPY, 20% VXUS, 10% bonds. Got rid of the bonds couple months ago and started going 80/20. Now I'm 100% SPY.
The thing I don't like is that I don't have VTI in my 401k. It's gain since inception is 413% while SPY is 1260%.. caveat is that SPY is very tech heavy. In a limit down or crash scenario I think those holding 100% SPY will lose a lot more than thsoe holding VTI due to the diversification.
It's why I'm trying to figure out how to add some of that diversification into my 401k.. but I don't think I have any real funds which can do that.
Look at every limit down instance in the last 100 years of the SPY. Did it recover? The answer is yes. The idea is you don't sell in a limit down situation and realize a loss, you buy. Buy, buy, buy.
It's why we say you need to leave you emotions at the door as an investor. If SPY ever goes to zero, we have much bigger problems than our 401k balances. And that is why you diversify your investments with guns and ammo.
And I say all of this as one of the more risk averse people in the thread.