I wouldn't count Taft out. Remember, the Truman administration was deeply unpopular, and therefore the election would be a uphill battle for Adlai Stevenson in any case. Even if Truman was personally popular, his party has held the White House since 1933. By 1952 incumbent fatigue had certainly set in. If given the choice I'd certainly prefer President Stevenson to President Taft, but I'm not sure the electorate of 1952 would agree. That said I can see Taft vs. Stevenson being considerably closer than the election that actually happened, since Taft is a far more partisan figure than Eisenhower was.
What really matters here, in my opinion, is who Taft chooses as his VP. Taft is still going to die in 1953, and so a Taft victory really means the Presidency of whoever Taft selects as Vice President.