Ah yes. Land. Until it's confiscated. Or taxed (window tax? Second home tax? Landlord tax?). Do you own the land? Or do you own a piece of paper from the government that says you own the land? A piece of paper whose credibility depends on the credibilty of the issuing powers, just like fiat currency.
Would you really want to bet your investment on a piece of land when you could instead bet on the US continuing to exist as a sovereign state? The only place you'd possibly bet on land would be London, maybe. Otherwise, I'd rather have my money in Treasuries.
A land grab by a stable government is not very likely.
Furthermore, unlike land that can give you valuable goods even if you can't sell it, US treasuries are worthless if you can't sell them.
I heard the following story recently, though I haven't checked if it's true... Back when the dollar was a gold backed currency, multiple foreign governments traded some of their gold in for US treasuries. It became clear to these governments that the US was issuing more treasuries than they had gold deposits for, so these countries tried to get their gold back. IIRC France was one of these countries. The US government stalled the sale of the treasuries then switched away from the gold standard.
If the above was true, France and the other foreign governments lost money in this deal. Who's to say what other adjustments could come in the future. So you see, US treasuries can potentially be a bad investment regardless of whether the US continues to exist as sovereign state.
I don't know all the details, but didn't a number of investors from all around the world lose money when the US switched away from the gold standard?
As for a safer investment, how about land ownership?