highly improbable at this point. but not impossible
truman was considered toast in '48. his opponent hardly even campaigned everybody thought he was so fucked. but yeah i don't think there's ever been a "comeback" of the magnitude trump would need
If I remember pollsters stopped releasing polls at one point in that election for awhile because they thought dewey was guaranteed to win that election. As a result they weren't able to track last minute shifts towards truman closer to the end of the election.The media during that election was also saying that it would be impossible for truman to catch up to dewey. Unless hilary pulls a dewey in the next few months you are right that trump won't become competitive again in this election.