Disclaimer: I am not an expert on any of this
That amount you pay is essentially the net present value of the principal at some low interest rate - basically you're getting $10k of your purchase price back each year, plus $2k interest (so about 2%). The amount is also fixed, so the purchasing power will reduce over time
Most western economies probably have higher interest rates than that right now, so their governments could get better UBI outcomes (financially) by paying themselves than relying on an annuity market
UBI shouldn't have to rely on financial trickery - even conceptually. There's more than enough to go around, governments should redistribute wealth in a way that achieves social welfare goals using the powers they already have e.g. taxation