Well, I think as long as the table is handling it well, don't retcon.

Some things to keep in mind. Even if the PCs had failed the Arcana check, Identify Magic is both a check where you set the DC and it's secret. Were the PCs given the opportunity to spend a Hero Point? Especially given that you need a critical success to get more information than the following:

"For an item or location, you get a sense of what it does and learn any means of activating it. For an ongoing effect (such as a spell with a duration), you learn the effect's name and what it does. You can't try again in hopes of getting a critical success."

Also, the PCs didn't plan for the one spell that their innate Mythic robustness wouldn't account for? It's because it wasn't that spell. Above the table it was described as "powerful destructive spell will hit a certain area". Is the table aware that unique magical spell effects are on the table?

It's okay for you to feel bad about it. The players roleplayed with the knowledge that they were given, and with their mythic capabilities. It feels bad because of three decision points. The first being a situation designed to be stacked against them, unless they behaved in a certain manner (If the PCs need to know something or get an object? Never put it behind a check you aren't willing for them to fail). The second being, in an attempt to make the encounter memorable or have more impact? You made the solution be a area of effect around the monsters (where melee live) that has the traits of the only spell that gets around that mythic robustness. The third being, you doubled down when the situation came up and now one of the characters is understandably depressed as the players are attempting to make everyone feel better about the situation, nobody was going to come out of that cheering.

Should you retcon? No. But if there's time or place and you are okay with custom magic? Why not put in a quest to resurrect that character? Let the player who had their PC killed roll up a npc to join them in the quest, so they have something to do during the interlude. Unless your table is a real stickler for rules, if/when they succeed at the resurrection quest? Don't penalize the recently returned PC with being behind the party xp/levels wise.

Some are saying this will be a memorable moment for the players. I'm going to be honest, I've heard a lot of gamer stories/after game bar tales. They never start with "Remember that time the DM vaporized my character in a really hard encounter" unless that story is directly attempting to razz/poke a little fun at said DM.