You seem to be missing the fact that Streamlined Progression will be a completely different mode than Easy, Normal, and Hard. It's not going to overlap the other difficulties and therefore won't affect anything negatively. If you want to still play the game on hard mode, cool, do that. And someone who just wants to experience the story and explore will play it in Streamlined Progression. It's not going to ruin your game experience regardless of how someone else plays. This is a single player game at the end of the day.
