Let’s go by parts.
I think we can agree legendary spells are frequently not the most powerful option. In some cases they are, but those are more rare.
By how much? Sure, it, in some amount, decreases the number of options you have to think of, but decreasing say, a 1/30 chance of finding a certain spell, a chance that will go lower as the expansions drop, is worthy of a new rule? Like , could you think of a case in standard where there’s a p roblematic interaction that is not soul mirror?
I mean, even soul mirror isn’t problematic, it’s just probably the most powerful effect that comes to mind of this interaction right now. So, what are we aiming to fix here? Especifically priest’s capacity to clear a board? Or is warlock Deck discovery problematic too? Is there even, historically speaking, a situation where discovering a legendary spell ruled the meta?
That’s subjective. It’s easy to understand why someone would find fun being able to find and play a spell that can, theoretically, turn the game. It’s somewhat wasy to see also, that it may be frustrating to get the game turned, but that’s more than a problem with discover than with Legendary Spells.
Let’s brainstorm it here: Which legendary spells are being, even remotely, run right now?
There’s soul mirror, there’s Evocation, and there’s Deck of Chaos.
Of those, when was the last time you remember losing to a discover from one of those? But on the other hand, when was teh last time you lost ot a discover of Fireball, Runed Orb, Apexis Blast, an early power word from priest on a sethek or bloodweaver or a Skull of Gul’dan? Yeah, i think it’s easy to see that Legendary spells aren’t that powerful, and even if they were, to have a problem with discovering something capable of overturning the game you would need to have a problem with everything that can be discovered.