I think both could be implemented together. In 5e there is exhaustion. Not sure if there's any plan to implement that here eventually
I believe this actualy is planned, since you can allready find potions that specificaly "removes exhaustion".

Either that, or is simply some magic debuff. :-/
Keep in mind light is tied to stealth. Diminishing lighting, could influence stealth breaking Larian’s existing design.
Oh that is true. :-/
Shame ... but understandable.