Allow me to present you a situation that brings this into light. Consider Ragnaros and Pyroblast; in my opinion, both pretty good cards, and both the same cost. If you were presented the choice between Ragnaros and Pyroblast in Arena (obviously not possible since they're different rarities, but just assume), which would you choose? This is not a bash on Pyroblast, but I'd almost always pick the Rangaros. Pyroblast lets me hit my opponent for 10 damage once; Ragnaros hits for 8 damage every turn and forces my opponent to trade AT LEAST trade cards 1:1 to kill it. Pyroblast, on the other hand, while a good finisher if my opponent is low, does not require him to expend any mana, board position, etc. to stop it if Pyroblast isn't the last bit of damage against him. And it only does it once. This should highlight the differences between Pyroblast and minions. And more importantly, it should highlight the concept of what a high-cost card is. They're ALWAYS powerful, and there's a reason for it.
I'd always choose Pyro. It's not random damage. It does more damage. It can't be sheeped before it hits what I want it to hit (if I get unlucky with the first random hit). At a pinch it can synergise with Archmage Antonidas. It can't be stolen or faceless manipulator cloned. Secrets (apart from 2 specific mage ones) can't affect it like all the rest can affect minions , etc etc.
Minions are not more powerful than spells, they are just multiple hits of weaker damage that can be killed while spells are higher damage but they are only one hit.
Minions and spells have their distinct uses, but I prefer a strong powerful minion to a strong powerful spell.
Anyway, consider the FULL aspect of what you're suggesting taking Pyroblast over Ragnaros. Mind I'll sub Assassinate for Polymorph just to be kind as it's more mana efficient.
1) Ragnaros. Even in best case for opponent, Rag comes out and either kills a mid-game minion (assuming you don't drop it at a horrible time) or does 8 damage to the opponent. Opponent then Assassinates it for 5 mana. If it kills a mid-game minion, it trades 2:1 on card advantage and gets a slight advantage in terms of mana advantage. If it hits the enemy hero, then it trades 1:1 in terms of cards and does 8 damage to the enemy hero in exchange for 3 mana.
And if the enemy can't remove it, it's basically game over within a few turns as no one can sustain the onslaught repeatedly.
2) Pyroblast. The ideal situation is that it deals 10 damage to the enemy hero and kills them. At a minimum though, the most common use is that if you can bring the enemy hero to 10 health, you can use it as a great game-finisher for only a single card. If you can't bring the enemy hero's health to 10, it's almost useless as it does nothing to swing the board, and in the late-game, that will lead to a loss within the near future. In a pinch, it can be used as an extremely inefficient minion removal card, doing for 8 mana what Assassinate does for 5.
Now, like I said, I don't want to bash Pyro as it does have its uses obviously. But if I had to choose one or the other in Arena, I'd almost always pick Ragnaros. The consistent damage every turn, even if random, and the fact that it will at worst be a 1:1 card trade with the opponent outweighs in most situations to be the idea that I can do more damage in a non-random fashion but only one time, and lose card advantage in the process.