The adversity of big fishing is the entire point of big fishing. It is not meant to be simple and trivial precisely because there are no consequences for it being an unkind challenge - there is nothing gated behind it and no reason to do it save to chase achievements and say that you've done it. And for a certain type of player.. that's exactly what makes it appealing.

Giving the rest of fishing more of a purpose might not be a bad thing, but it doesn't have to come at the cost of one of the quirkiest and most interesting long-term goals in the game.