Powell was at the end of his appointment already, it’s a fixed term. He hadn’t been removed early. The president appoints the replacement. The investigation didn’t do accomplish anything.

Its actual goal was probably to pressure Powell and the Board of Governors into setting rates that Trump liked. That also was not accomplished. In fact, it failed so epically that it looped back around and was preventing Trump from getting Powell’s nominated replacement approved.

You’re right about the investigation not starting back once Powell is gone though, but because pressuring him won’t help pressure the Fed chairman because the Fed chairman will already be a Trump stooge.