The reason for the assassin meta isn't because they're particularly overpowered.

People run assassins because tanks are way too strong now. Literally no one runs a team designed to kill the tank anymore because they just flat out lose. Mages kill themselves on Hilda even with anti-reflect chest and do literally 40 damage to Christiane. Clarett doesn't do shit vs modern tanks and Mystery Knight is too easy to counter and too hard to fit into a box.

Make more viable tankbusters like Wataru and then you'll see tank-push come back. Until then, the only viable way to win a game is to find some way to bypass the tank through AOE, assassination, or displacement.