"Bad" is always better than "worse." If FDR had decided to not run, the most likely candidate would have been Henry Wallace; he would have handled the war similarly to FDR, but would have given Stalin everything he wanted after the war (remember, he was the 1948 Progressive Party candidate). If Wallace had been defeated by Wendell Wilkie, Wilkie had left the Dems because of FDR aggrandizing power, so he was the original assistant Democrat. His famous 1942 book? One World. All you need to know.
that already happened. When we did not enter Berlin and allowed the Soviets to occupy it as we stood back it was a disaster that cost us the rest of the 20th century.