> any change resets the entire contents of their brain
That's because it does. Consistency is incredibly important.
The problem isn't that you're adding a feature, the problem is that you're adding a feature in an obtrusive way. Add as many features as you like (while preserving performance), but keep the day-to-day UI as stable as you possibly can. Place entry points (buttons) for new features in menus first, and make sure they're both used frequently and by many users before moving them to a crowded toolbar (and then give good thought about where it belongs on said toolbar/menu). Don't remove features unless they're truly problematic, and don't change UI.
That's because it does. Consistency is incredibly important.
The problem isn't that you're adding a feature, the problem is that you're adding a feature in an obtrusive way. Add as many features as you like (while preserving performance), but keep the day-to-day UI as stable as you possibly can. Place entry points (buttons) for new features in menus first, and make sure they're both used frequently and by many users before moving them to a crowded toolbar (and then give good thought about where it belongs on said toolbar/menu). Don't remove features unless they're truly problematic, and don't change UI.