The more they flesh out Sparda's past the more they ruin the mystique of him, to be perfectly honest.
100% agreed. Move forward, keep him as a legend, and surpass him. I'd rather it stay a mystery.
This isn't like MGS3 because Sparda is a hero of a demon, where Big Boss was a villain. There's nothing of a surprise to reveal, just a retread of a legend told in two different prequels (of which, one isn't cannon).