If they felt there was a problem with the scoring, the Koreans should have protested right away. The rules say the protest must be made before the end of the next rotation. After that, no scores can be changed.
We don't know for sure if this particular mistake would have changed the outcome. Maybe there were other scoring mistakes among the many other routines by other gymnasts. But the only one that has been retroactively examined is this one score by the Korean.
If they were going to change the outcome now, the only fair way would be to reexamine the scoring for everyone for the entire night. There's a reason they don't do that. It would be like using "instant" replay on entire football games and changing the score after the game has been over for days.
When it's over, it's over.
Coincidentally, the Japanese team won the gold.