The only reason why you don't use rain of fire in mono fire decks isn't because it is weak, but because it is expensive...
In a unupped mono fire, you take heavy (and expensive) hitters like phoenixes and dragons (or lava golem) and add some cheap control that won't slow you down too much. (deflags+rage potions or fire bolts) A Rain of Fire would be really strong, but expensive at the same time, so you won't use it in the rushy fire element that much. (Arena decks are a different matter (since double draw/mark increase/etc.))
Rain of Fire, an expensive control card, just won't fit in the normal picture of the fire element with strong hitters and cheap control.
I personally don't really care if the unupped cost is decreased to 6, but anything else would make this card too strong.
(And by the way, the ratio between damage/cost of thunderstorm and RoF is fine, since you can control many rush cards like shreekers, or frogs, and some others with a RoF, but not with a thunderstorm.)