I'd much rather see resistance revert to prior editions' "subtract a flat number from the damage" (which lets fire resistant creatures do things like sleep in hot coals, or cook food without utensils etc), and then remove immunity, replacing it instead with "really, really high resistance".
So sure: you CAN hurt that fire elemental with fire. But he's got enough fire resistance to comfortably have a bath in lava, so you're going to have to have a heck of a hot fire to do anything to him.
I totally agree with your point. Fire should be a weak choice against fire elementals and devils but it should be a choice.