This is pretty much exactly how i feel. Thank you for putting this into a well thought out post(which i couldn't do). FF12 is an exception in my book, i think that game has an excellent balance of style and modern english speaking techniques. The voice actors they chose for that game also made things easier to understand as they were bloody brilliant IMO. My favorite among the bunch being "Cidolfus Demen Bunansa" voiced by john lee. One of the reasons i think they did a particularly good job is that most of the retro style english themes they were using were performed mostly by nobles, I.E ash, balthier and fran. Fran being an exception as isolation was the key to her races survival. She is also like 500 years old IIRC?, so it would make sense for her to talk that way, this is why i think XIV needs to have a good balance in the story of ARR.
Everybody has their own way of speaking yes. You have the hard workers at uldah, who speak like we do now, with some quirks here and there. But then you have the scions. Who ALL speak in riddles...when they should be the ones speaking in a more modern tongue. Lalafels speak this way because of their parents right?, so i think they're ok. Sure giving the scions a bit of a unique style is always good for story, and to get people entranced for those use to it. But it get's really annoying when the scions voice actors suck...example. When you are about to get into a massive fight, and minfillia or someone else says, "doth thee knowest no bounds?" "hath thee no sympathy for the weak?" stuff like that is..so blegh with bad voice acting.
Video of cid being awesome - https://www.youtube.com/watch?v=50tMUysAXm8 quality is a bit on the wild side, you will notice a slight dip in the voice. This is due to an old emulator issue.