Pure fighters stay viable imo.
The whole “fighters get weaker, casters get stronger” thing is more of a table top issue.
Like in PnP a level 18 Wizard can reshape reality. Gravity? He controls it. Planes? He can make his own and travel them at will. Time? He can control it. High level divination spells unfold reality. What is the enemy doing? Let’s pick apart reality and find out. Fucking Imprisonment. WISH.
In CRPGs Wizards tend to get literally none of those spells. Heck you can’t even fly above the battlefield at 120 feet raining fireballs on everything as it helplessly runs for cover. Now granted something like Wail of the Banshee can be utterly insane. Doing 180 damaged to everything within 40 feet is bonkers. But so is a fighter going after a single target and dealing 400 damage a round or whatever.
Also Mythic Paths narrow that gap even more imo. Mythic Power attack and similar feats scale damage to frankly absurd levels. Now is that better than all my spells on my lich being like DC 36 with +20 spell penetration? Maybe not, but it’s more than viable.
The more likely problem is that it might feel “boring”. You run up and hit stuff. Really, really hard, but still just hitting stuff. Again Mythic Paths help here, giving you wild and crazy abilities that aren’t just hitting things. Maybe you move up and then summon something or drop holy fire on stuff. And then you start hitting stuff.
Edit: Just an example:
Imagine this is most CRPGs. “We don’t know where the bad guy is/what he is doing,” is often a key part of a narrative. Discern Location tells you where he’s taking a shit. So you can pop 35 spells, teleport to him and drop everything in the universe on him before he’s wiping and then be gone in a blink if he somehow survived (probably some version of Contingency so you don’t even have to be able to react yourself) . Make sure to use some other scrying method to see exactly what his shitter looks like so you can assure no surprises.
Contingency is another one that’s insane. “If anyone ever attempts to harm me, teleport me to my safe room in my personal pocket demiplane.”