The investment bankers lost money even before the transaction completed. Interest rates rose while Musk dawdled and delayed buying Twitter, so the debt lost val

The investment bankers lost money even before the transaction completed. Interest rates rose while Musk dawdled and delayed buying Twitter, so the debt lost value (this is the same issue that drove some banks into bankruptcy is the debt they owned - bonds - devalued as interest rates rose). Ordinarily what the investment banks do is sell the debt and then enter into other deals, but with the Twitter deal they either have to take a bath and sell the debt at a loss or keep their money tied up as the debt gets paid off.