Buy US treasuries? Those rates are still positive (0.09% - 1.66%). Since the capital is essentially free even if you get a blended rate of return of say 0.2% that is still real money. As an example USDC has $5.1 billion dollars in outstanding tokens issued that's $10 million a year at 0.2% just to sit on it.
Sort of; that has a risk you haven't accounted for: borrowing long-term and lending short-term (or rather, no fixed term). This is the same kind of risk that absolutely savaged WeWork.
For instance, if we take your 0.2% blended rate estimate for what you can get for your capital, that's still a real yield of -1.8% accounting for inflation. Luckily that 2% inflation loss is born by your token holders, not you, however that creates a real incentive to exit the tokens once interest rates rise.
Let's say interest rates rise to 5% due to a financial recovery - back to where they were in 2010. Now, not only have your token holders lost 2% value for each year you've carried the T-bills, you have to discount the notes by 3.375% (vs the current market 1.625%) to liquidate them should your token holders decide to redeem and move into bonds themselves.
To liquidate them you have to pay the difference in coupon rates, so a mark to market loss of $172M. If we take your 0.2% blended rate as an offset, even over a 5 year window, that still represents a net loss of $122M.
Borrowing long and lending short works until it doesn't, then you can ask Mr. Neumann what happens.