I mean I'd argue it is a balance change, as you're adding either a minimal resource drain or a "Woops you didn't buy a torch guess you've got to turn around now and go get one". Its not making the dark threatening again, its making the dark annoying again.
Mind, having both seen Skyrim players have a dislike to all the "Make the dungeons dark so you have to use torches" mods after the fact, and played a lot of Ark Aberration (the cave map) where colour balance is a god-damn nightmare during the dark moments. The dark ain't a threat in Aberration, but the radiation, giant murder-snakes and the hellspawn (who stay away if you got a little light pet on your shoulder) are
Gritty realism absolutely impacts them? It slows down everything by its nature. If they're not expecting that slower pace and came in for the default chips and nachos 5E experience, why wouldn't they complain?