But each one has a maturity date. If you still own it on that date, it goes away and you get your $1000 deposited into your account, which you can use to buy a new one. Or not.
If all you own are the bills, that means that none of them will last a long time before they turn into cash - they aren’t as risky (if you think owning US Treasuries are risky).
A Treasury bill matures in 1 year or less from when it was issued. A Treasury note is more than 1 year but up to and including 10 years. Treasury bonds are anything more than 10 years.
Since, within that timeframe, the risk of Trump crashing the stock market is far greater than Trump having the US default on its debt, Buffet is making the smart move.
My impression is that he started that process once he got a whiff of the possibility of a second Trump term in combination with the likelihood of an incoming recession (based on historic indicators), no matter who won the presidential race.
That's smart as opposed to well connected / insider trading.