The declining health of FDR was kept secret at that time. It was only after he passed away, that we learned he had been in declining health.
Except, it doesn’t matter if Hitler is thrilled. A country of the people and by the people, does not need FDR, we need the best second in command you can find. Wasn’t it Truman by them? I imagine a better outcome with him in a position of POTUS. He took not guff and was tough as nails.