Ranking voice casting from F-S to give you an idea of how this works in my mind,
When the base UNI cast is a solid A at the lowest casting to S with some other characters, I feel rightfully annoyed and downright angry that people want to tell me the D/Cs of the English dub are a perfectly acceptable alternative, when their very best fall two points below where I'd place some of the most lackluster of the Japanese. And keep in mind, I'm being generous with D/C, If I'm going to be honest it's more F-C and there isn't a single D.
Holding open auditions and taking a bunch of people who have no acting skill then putting them into the project is in no way not ever no chance no how going to ever hold a candle to the complex and needlessly rigorous route that a JP voice actor or actress is going to take before getting somewhere, and the amount of experience and skill in both manipulating their vocal chords and just being able to act that comes with that route.