attack doesn't matter against a mob with no defense buff for dmg
you could have 10million atk and the number wont change
there are vastly diminishing returns. technically, the more attack you have, the more damage you will do, the point in which you will stop noticing is when the increase in damage is less than 1 point. would post math, but already kinda off-topic.
the reason we stop statting attack and go for damage after a certain point is due to the "softcap". yes, your damage will still increase, but at a lower rate than either you are willing to pay for or less than it would increase by instead increasing a different stat(i.e. damage)...