I just went on vacation to upstate New York and stopped by the FDR library and museum in Hyde Park. In it, the tour guide claimed that FDR was felled by a cerebral aneurysm with no warning. It had nothing to do with polio or anything else. He was only 63 or so at the time IIRC.
So, suppose this freak accident does not happen and his presidency continues into the fourth term and possibly beyond. He may not have been in perfect health outside the polio, but we can assume that he gets the best health care the United States has to offer (similar to Trump). As a result, let's extend his lifespan to 73 or so.
How do things transpire from this point on?