Yeah, I think if everything else remains the same then adding a mythic martial just somewhat levels the playing field for those that want to be martially and play at high levels next to the current jacked up Wizard.
We already have jacked up Wizards which basically violate most fantasy genres anyway.
For me:
Perfect world = total redesign of spellcasters, and martials should be made more interesting but don't have to go gonzo
Assuming spellcasters remain the same = it's the bad design we have and leads the high level game into gonzo supers territory so just add a martial equivalent (keeping the existing FIghter). It gives players that want it a chance to play that kind of character. And since we can play all caster parties now anyway, it will not "double down" on broken. It's the same broken with a happier sub group of players.
And honestly, I don't want to redesign spellcasters. I've seen all caster parties, they can be a ton of fun. Yes, it means you aren't playing Game of Thrones or Lord of the Rings... but I don't WANT to play those, I want to play DnD. A game where a Runt Pit Fiend can adventure with the daughter of an Archangel into a crashed spaceship and use a laser rifle to shoot legally-distinct Cthullu. Something you could do in SECOND EDITION.
[Edit: This turned into a bit of a rant. I do want to say this isn't directly aimed at you Bert, it is just a general frustration]
Look, I get people want serious stories. I do to. But having high power or even silly sounding concepts
doesn't mean you can't have serious stories.
One of the most deeply touching moments of friendship and emotional story-telling I have ever witnessed involved an entire season of a show following the kidnapping and rescue of a woman. We saw how her entire island, every member of her family, her entire culture was destroyed when she was eight, and one of her only friends dying to save her. We saw her hunted by the world government, forced to run and fight as people turned her in for the bounty on her head. We saw her give herself up to the government to save her friends, just waiting to be executed as a small man without talent tortured and beat her, until her friends arrived and demanded she be honest that she actually wants to live and not be in fear for her life, and declared they would fight the entire world if that was what it took for her to have that.
Those friends?
A man made out of rubber. A man who wields three swords by holding one in his mouth. A man who uses a kid's slingshot as a weapon. A woman who uses a colorful rod that makes weather. A man who kicks people because he's a chef and won't use his hands to fight. A cyborg in a speedo. A musical skeleton with an afro. A reindeer that shapeshifts into a monster.
It is a silly, ridiculous group of people and concepts. Yet you will never convince me that the story they told was not grand and epic in the truest sense of the world.
It is endlessly frustrating to me that people take this idea that they could never take a story set in a world with a high power level seriously. High power, even if it is "everyone can fly and shoot laser beams" doesn't remove your ability to tell a story about friendship, or about loss, or about grief. Serious stories tackle serious issues. A story about loss and grief isn't better and more impactful if it takes place in France during WW2, in the depths of the Mines of Moria, on the Starship Enterprise, or on the planet Amazing Bubblegum. I know people who refuse to take any "silly fantasy story" seriously, and it makes me frustrated the same way that this does. Because it derides story-telling by declaring that the only stories that can matter are those that are non-fiction and like our real world. Being closer to the real world doesn't make your story better. It doesn't make it more artful. It just makes it take place in a world like the real world. That's it. You aren't a more cultured person enjoying more cultured stories just because you feel like the story you are consuming "could really happen".