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How can the devs ever hope to balance a game between 100% exp gain and 600% exp gain?
Pokemon ALREADY throttles the difficulty totally unrelated to exp. When was the last time an npc actually used more than 4 Pokemon? Or switched out their Pokemon during a weak matchup? Or had a team with any sort of synergy? Exp and difficulty are not nearly as connected as you think. There already exists other ways for the players to challenge themselves.
This game has a huge amount of in-game species (not to be confused with national dex) and a heavy focus multiplayer features. That creates a huge need for players to have lots of levelled Pokemon or variety goes WAY down.
"Muh choice" people will say. You want to play the game at 16% the designed rate? People are gonna try it and hate it, and poor whoever child that turns it off unknowingly and never finishes the game.