And yet it is the low-tech, decidedly corny and overacted (Heston) film that

Corny, yes. Overacted, no. I've seen this criticism made about Heston again and again, and not just for this film. Heston belonged to a different, older, school of acting, one more beholden to a mix of theatre and wide-screen drama that decidedly does not play well on smaller screens. It is only 'over-acted' by comparison... which I think is unfair.

We are on the other side of Marlon Brando and Francis Ford Coppola now, and we often don't acknowledge the change that was wrought around those careers: naturalism, interiority and a kind of celluloid intimacy rules now, where before the drama was done up in bolder strokes, wider vistas and bigger gestures. I don't think either 'style' (if style even be the proper word) is better or worse than the other.

And, for what it is worth, I disagree with your premise, re: the original surprise ending. You should seek out a 1938 movie called "Angels With Dirty Faces" with James Cagney, Pat O'Brian and Humphrey Bogart. Cagney was fencing with questions about the ending of that one, and his acting choices, up until his death fifty years after.