Taft might be better able to rally conservatives but would lose moderates and indies. Truman wouldn't be picked in 1940 because he was still the Senator from Pendergast, an opinion which FDR and the Dem establishment shared. Plus he had an excruciating primary which he only won by a few thousand votes.
^^That's true. FDR had run with Wallace or Byrnes.
But Taft would have underperformed the charismatic and liberal Willkie.
Franklin D. Roosevelt/Henry Wallace: 481 EV. (57%)Bob Taft/Arthur H. Vandenberg: 50 EV. (41%)