I would be less concerned now--but was extremely worried about this in '06 (terrified, actually), so much that I switched allmost all cash in the 403b to Fidelity Intermediate Treasury--which turned out in retrospect to be one of the better change of allocations I've made. Shifting most of my large cap growth gains to small/mid value from 2000-2002 was the other, although part of that was rebalancing, part of that was changing the allocation to include more value, and part was terror at tech and S&P PEs.
The shift at both firms to government paper for MM is a good thing, even if it means no yield.
In early '06, we had some bonus money from DW that we were putting away for the yewts' college and the Schwab guy tried to get me to put it in their "yield plus" money market (I can't remember the fund name), which I refused and then read the prospectus to see all the bank paper in it, including several banks that went down in the Crash. In '08, it and its co-fund sank like a stone and Schwab was in litigation for many years, since they had sold it as a "safe" money market--Allegedly, to be sure, but based on my experience, I believe it. It turned out we didn't need it for the college--we paid as the yewts went, but it could have been devastating.
(Also, I think the switch to primarily govt paper was pretty much mandated in the post-crash Reforms/regulation--but maybe not.)
Cash is 12% of the investment portfolio, but more (16%) if you include savings. Too high, but I'm OK with that. I am putting a bit more in stocks/bonds incrementally when the market goes down 3%.
I don't consider commercial paper money market funds all that safe. There's a reason the brokerage firms switched their settlement accounts to government money market finds.
I hold cash at Fidelity and Vanguard in the treasuries only MM accounts - FDLXX and VUSXX.