I am aware one may buy more Archmage's rings, but that is my point. Even if there is not a unique for Yeon and/or Kyrie (I don't know if it is even possible to max Yeon's affection) there is no need to buy more, because they are only two options to even equip non-unique relics on.
It seems like a nuclear way to address power creep of casters, especially given that this reality is not limited to affecting caster units. Yama is just as "penalized" by this mechanic as are casters, so it does nothing to mitigate caster power creep relative to other units. This does nothing to alter the state of late-game caster's power, other than making people drink more mana potions, which procuring is not even an issue in late game other than being an annoyance. One could, for example, increase resistance of late-game enemies, or have the lvl gains of MAtk diminish above a given level, if the desire is to actually make casters less OP in late game.
But really, "casters are weakest classes at low lvl, and strongest classes late game" is ingrained logic in every RPG in the last 50 years. That is just the way things go, and it makes sense from a world-building perspective.