You seem to be assuming that the heirs of billionaires are going to sell everything eventually, but I see no reason to think that should be true. They can live off the interest forever and the state will never touch the principal.
Also, couldn’t you make the same argument that wage income, and every other kind of tax should be deferrable if the money gets invested? We should all just not pay the government and the government should give us an IOU until eternity? I suppose the modern monetary theory people would be on board.
The difference between this and deferring wage income is that wages don't need to be liquidated. And that's not a small difference.