Firing Fed Chair Powell would be a highly destabilizing move, drastically increasing the perceived political risk of investing in U.S. Treasuries. The resulting pullback in foreign investment would place immense pressure on the U.S. government's ability to fund its operations, demanding much higher interest rates to attract any investment. Since Trump is expected to demand interest rates to be lowered, it's unlikely the U.S. government will receive the funding they need to continue operations.