Wall of fire DOES kill creatures, it does more damage then the appropriate damage/level table during the levels that you are assumed to be using it.
3d6+int per round is larger then 2d8+5 I also disagree with the example. I don't think the braizer should be a high damage option, however I would give it high damage because it is coupled with the swinging kick, after that I would consider it low damage, or medium at best if it's a large braizer.
It takes some time to get burnt unless you are surrounded in flame by a bonfire, and you will be wearing some clothing that should protect you from the flames to a reasonable degree, you are adventurers after all.
Sure wall of fire doesn't scale, should all the powers be based on this table and have them do "limited damage: high" then I would feel like leveling up had become pointless, except to gain class abilities.
Ultimately people like to have some tangible advancement, damage/attack bonus things all seem to scale, but you get more powers, more options, more class abilities, more everything as you level. health/attack bonus/defenses. These things only scale (imo) because they show a difference between higher and lower level (then you) monsters, lower level being easier to hit ect. Again showing that you are getting stronger. You can go back and fight that kobold army when you are level 10 and it's obviously a much easier encounter.