I'm guessing that US Treasuries are about as safe as anything, for US citizens living in the US. I would guess that Warren probably agrees.

I would tend to agree, especially for US persons. Other than physical banknotes, I can't think of anything safer than T-bills in your shoes. The only small incremental safety improvement would perhaps be a small admixture of banknotes of another currency, just to round things out?
My own hesitation on them is probably 90% about whether I want my money in the US period, and only a little about potential investment risk. Plus, if I'm not holding US cash to be ready for doing US investments on short notice, what do I need US dollars for?

A friend of mine raised the thought of picking up some gold as disaster protection. Not the sort of thing to get a great reception at this board (for sound reasons), and given the recent price run-up he's also a bit late to the party. But I notice silver is still below its highs from the last cycle while gold is twice as high. Using options on SLV, one could possibly even create a tiny income. A repeated covered call kind of thing. That gives up any big upside, but it's an asset from which you don't expect any lasting big upside, just noisily treading water forever. The problem being that something dependent on elaborate financial strategies might not be the thing you want to rely on in a disaster scenario, depending on what sort of disaster you're trying to prevent. Financial crashes, wars, or asteroid strikes?

Jim