"Yes, but if we got rid of every tower-attack card that doesn't hit as hard as Dark ritual, we'd have none left. It doesn't mean that those that strike for less are any less useful in their own right."
But Dark Ritual and Earthquake serve the same niche (at least if we take it to be true that, as you said, Earthquake is a finisher), have about the same cost, have the same rarity, and one of them is better by a wide margin. That seems wrong to me. I think cards of the same rarity should be about equally powerful.
To take a contrived example, I think it would be poor design to have, say, an uncommon no-keyword card that does 15 damage for 10 recruits, and an uncommon no-keyword card that does 18 damage for 10 recruits.
Now that I think about it, I may have spoken too quickly in declaring Earthquake the worst (which says a lot about the balance of certain rares). There are two rares that are definitely worse. However, I'd put Earthquake squarely at third or fourth, which still places it in "Never going to take in my deck in a million years" territory.