I’m strongly against this idea for many reasons.

This is supposed to be a new game, with new ideas and new features. It’s been steering away from the old games, and it’s been working out well. Second of all, adding this feature would be unfair for many people. Some people living next to or close to bodies of water would be able to go “fishing” and have access to a lot more magikarps, feebas, barboach, etc, as appose to someone who the closest body of water is 10-15 miles away, and won’t have the same opportunity to farm candies for those certain Pokémon.

Lastly, the feature doesn’t fit in with any other parts of the game. It would be weird for it to just be there, and would just be out of the place.

The main reason is that it’s not fair. It gives some players too much opportunity, some little, and some none. Just a bad idea to have this added.