The problem with the EXP Share is that people are always going to gravitate towards making use of whatever advantage they are handed, as a game designer you're always going to have to assume that people will take the easy way out, even if it hurts their overall experience. It's also impossible to tune your game's difficulty around it, what I experienced in Gen 7 was that when you have it turned on the game is way too easy, but if you turn it off you have to grind way too much (it's not difficult, you just end up falling behind), it means you constantly have to switch it on and off to try and hit the right sweetspot, and it becomes a bit of a messy experience as a result. Hiding a difficulty toggle behind several UI screens is also pretty bad, although it's obviously done so kids don't accidentally turn it off.
I really like the idea behind being able to choose your difficulty in Pokémon, but I don't like the execution at all, it's just not very well thought out. I see no reason why we can't just have a good old fashioned 'choose your difficulty' when starting a game, as long as it only affects the main story. But then again, we're not allowed more than one save slot either, which is another baffling decision, so that might be why.