"Psion" always sounded weird to me. Were they just trying to give it a name that wasn't as mundane as "psychic"? Mystic seems to work for me. Mysticism invokes feelings of perfecting mind and body, uncovering mystery, transcendence, meditation, and becoming awakened. That seems to fit psionics well. Gives me some context for the power instead of just vague brain-magic.
I dunno, does everything have to be controversial? The official name for the class is one of the least-important things one could possibly take issue with. I mean, it's not like they won't give a bunch of suggestions for alternative names in the final class descriptions. I'm just finding it kinda strange how so many people can care this much about a single word in a rough draft of a fragment of a playtest version of a class. Like, plenty of people still call the rogue class thief. Why is this so important to get "right"?