The answer to the question is Duke Snider, who hit the last home run at Ebbets Field before the Brooklyn Dodgers moved to LA. Tell you who else this closely describes is Mickey Mantle, who never had a stretch of 40-HR seasons like Snider but also had a below-par end to his career that left his career average at .298 and went his last 4 seasons without hitting even 25 HR. But Mantle never left Yankee Stadium, either. When I first read this I thought it might be Willie Mays, but he never had a stretch of 5 seasons in a row of 40 HRs (he was close), and he only spent 2 seasons with his new team the Mets and ended up with an average of .302.