On: Billionaires Have Sidestepped a Tax Aimed at the Rich

Because inflation ≠ inflation and what do you want to index it to?

Seriously: If it's related to investment, what makes the price of tomatoes more appropriate than the Dow Jones index as a basis for the threshold?

One good answer there is to trust your future self to choose better than some simple mechanism would do. So that's what legislatures often do.