I think that it's also the fact that the Remake is going to be working really hard on how to portray those characters at the start of their journeys, which may be different from what the voice cast who's been with them all this time was able to offer them because they end up being a
version of their portrayal of that character, whereas new voice actors would completely settle in to just being the character and developing throughout the process
INTO something more like the versions of the characters that they're familiar with.
I see this as being somewhat similar to how they bring on a lot of people who grew up as fans of the game on the development staff to be able to bring a new genuine reimagining that still holds on to the specifics that people fell in love with. One of the other things is that a LOT of the other SE dialogue sounds like anime character dialogue, vs. things in FFXV where people started sounding a little bit more like people and not just characters, and that Remake is REALLY pushing that sort of natural communication between everyone, and that's something that gathering a new group together is likely going to help encourage, like it did with the Chocobros.
It's nice to see a scenario where the VAs are all being really supportive and respectful towards one another, because not all situations shake out this way, and sometimes it feels like a bunch of shady backstabbing (Evangelion redub for Netflix comes to mind), and I'm just happy that this doesn't seem to be that at all.
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