Kylo’s abilities could have been below Darth Vader’s without any trouble to the plot. If this ST happens in a sort of “dark age” after the fall of the Empire, then all abilities should be somewhat diminished.
Like a scientist in the Middle Ages, Kylo could have been inferior to all his predecessors, have worshiped the knowledge of older ages and desperately try to reach it while still being considerably superior to his peers in the time he lived in.
It’s not an objective failure of the movie though, but more of an absolutely subjective I would have done it differently.