I've said this before and I'll say it again. Hit them where it hurts most--their wallet. PSA: Your old gears including obtainable items in game are more than enough for you to one shot most monsters in this game. Why buy OCP to be stronger when there is nothing stronger to kill?
Just by increasing POW stats (or SPL for magic users) are good enough to simply amplify your offense power without the need to use any updated OCP for 4th classes