A lot of people want a harder difficulty option. I've never seen anyone suggest replacing what's currently there with something difficult.

Of course not. It'd ruin what pokemon is known for- the diverse community. 

If you think Pokemon is easy, then come online and play battles against other people and you will see.

It's incredibly deep and challenging. With an enormous amount to learn and a lot of time to train pokemons to a competitive level.

Pokemon is on the easier scale for me quiet some time, same with Final Fantasy and few other JRPGs. Typically I find Dungeon Crawlers to be the most challenging, so if I had to make scale between 1 (which is what I find Final Fantasy on) and 10 (What I find some dungeon crawlers on), I give this between 0-2 sometimes borderline babying you, and sometimes it picks up towards the end but never game over. 

The campaign isn't, which is what this thread is talking about.

It's fine, they somehow make it doable for both beginners and competitive experts. Thetotem battles were hard af and the Pokemon league was hard too

The correct difficulty is whatever difficulty the developer chooses. Nintendo wants the story in Pokemon games to be easy thus accessible and beatable by anyone, so the games are going to be easy and only when a Pokemon game's story isn't easy by design should the developers be held at fault.

I found the story in Sun/Moon to be very easy so everything checks out!

I think difficulty is an integral part of fleshing out a game, and some games don't gel well with certain difficulties. Dark Souls shouldn't be easy and Kirby shouldn't be hard.

I don't think the decision should always be left up to the player.

They are childishly easy. I know the main challenge comes from battling other players but the trainers and even the gym leaders are brain dead.

Pokemon seems to be a game that would work as either easy or hard. Unlike Dark Souls/Kirby, it seriously works regardless of whether the game is insanely hard or not.