That is just not the case.
If FDR had stepped aside, Henry Wallace would have had the Democratic nomination and would have won the election of 1944 and that would have been a disaster.
Instead, FDR dumped Wallace and picket the much more conservative Truman to inherit the presidency.
Roosevelt could not stand Truman.
The Democratic Party leadership new that Roosevelt was not going to live through the 4th term and did not want Wallace to be president.
The party insisted on Truman.
Why?
Answer: Because FDR picked Wallace in 1940 to be his VP, at a time when he was aware of his disease and the prognosis.
If FDR was not aware of how far left Wallace was, then he was not as astute politically as one might think. If he was aware, then the contention that he dumped Wallace in favor of a more conservative Truman is weakened.
On the other hand a lot of things changed in the world from Nov. 1940 to Nov. 1944. Perhaps his dumping of Wallace was due to a more realistic view of the world.
Maybe if these control freaks would let voters elect whomever they want when they want, voters would have learn a lesson with Henry Wallace and we would not have Obama now!