In order for some people to beat the investment average by timing the market someone else must underperform the average. Active investment management is in general a zero-sum game in which you compete with a bunch of math PhDs at hedge funds that do it as a day job (and even they have a tough time). Of course you can win if your own analysis of when the conditions are right is better, but trying to do so is usually not recommended for individual investors.
Holding cash sounds simple but if it goes up from here how do you know when to buy? If it goes down from here how do you know when to buy? And if you never buy, over a couple decades you will almost certainly do worse (Japan is a different case because at the bubble peak their stocks were measurably more overpriced compared to almost anything else in history including where the US market is now). In the end I think the only simple thing is the advice most economics professors would give which is to give up trying to time the market and just buy a low-fee mixed stock/bond index portfolio (with the ratio based on your risk tolerance).
So if someone has $100k to invest, maybe they buy $10k every month of their favorite index fund over the course of the next 10 months. If prices continue to drop, some of that money will buy at lower prices, but if they rebound over the next month or two, at least some of the money will buy at the (current) low price.
Then there's also the possibility of a dead-cat bounce somewhere in all this later on. Even if it looks like prices are going back up, they could drop again, and -- again -- there's still more cash to put in at various prices.
> Bear markets are paved with the blood of optimists.
I agree with exercising caution in the sense that trying to time the market is difficult and probably inadvisable in general, but I don't agree that people should be careful specifically because of the recent drop.
IMO the time for caution was weeks ago when the market was much nearer all time highs and it was clear the Coronavirus' growth rate was not under control (and that the measures necessary to contain it might reduce economic growth). Now is a considerably worse time to exercise caution.
People investing for the long term should probably have equities as a substantial portion of their portfolio. If someone doesn't already, I think it would be a mistake to hold back just because the market has dropped. The market may go up or down from here, but the alternative investments of cash and bonds aren't obviously better choices (and it's expected that stocks will be pricier than they were in the past if bonds return less than nothing after inflation). I think a lot of how things play out will depend on future political decisions around monetary and fiscal policy that are not realistically predictable.