I feel like with the exception of certain end of a world dungeon bosses and advanced combat. Most bosses in this game should be stunnable. It makes no sense to ban stun spells pvp wise... but also make it a useless PvE mechanic that doesn't contribute to PvE almost at all.
There is no use lowering damage of spells pip cost wise to add a not so useful outdated mechanic that isn't very beneficial pve or pvp wise. Why stun mobs when the strat for mobs are usually just blade up and aoe damage. Bosses have a larger health pool, but almost all of them aren't stunnable. I'd rather give up the stun for other utility on a damage spell to hit a boss.