Given FDR's health in 1944, he probably was beatable, but it just would have taken a very special candidate.
Dewey had some major problems as a campaigner, which were exposed in 1948.
FDR really was only unbeatable in 1936 and maybe 1940 (though that third term is a pesky thing...if World War II wasn't in progress, I'd argue Wilikie probably could have beaten him on that alone.)
I actually think he was more unbeatable in 1932 than 1940. Hoover was DOA that year.
Hoover was DOA, but FDR easily could have not been the Democratic nominee for President.