There have been a number of different incarnations of the Mystic in D&D, none of which has had anything to do with Psionics.
In Rules Compendium D&D, "mystic" was the name given to the monk class. This is a class which has been associated with psionics in various ways over the years: the popular monk variant in Dragon 53 stated that "The new special abilities and powers presume that the inner strength of monks flows from the mind - that it is a sort of psionic power." And in 4e monks drew on psionic power.why use a name that was used before in a completely different context?
Also, githzerai have long been described as cenobites
Wizard, sorcerer and warlock are all synonyms - and could encompass a D&D bard, druid or even ranger - but the game gives them a precise meaning. I don't see Mystic being any different.Mystic could describe anything. It could be a Cleric or a Wizard. Or a Paladin, a Sorcerer, even a Barbarian if you go with the animal totem thing. I could understand the desire to get rid of Psion and Psionicist (although personally I think they're fine) but Mystic isn't a good substitute. Might as well call a class "Adventurer".
Wizard, sorcerer and warlock are all synonyms - and could encompass a D&D bard, druid or even ranger - but the game gives them a precise meaning. I don't see Mystic being any different.