This is the problem: if you don't cap the completionist he is going to overgrown content especially in a non linear environment. If you want to cap without having a hard cap you could have a diminishing return per area albeit it would shift from the 5e of XP per monster killed.
Your point is self addressing like I mentioned players going the harder/faster route are automatically handled. The issue is coming from the completionist type that would trivialise content because they simply want to explore everything.... You don't want that. Alternatively you could have monster hp/dmg/number scaling based on the player level so the completionist is still challenge (that's what a DM would do). But you will want some cap to reduce the extremes.
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