They cannot, and so their position is already compromised, and it bleeds into dozens of threads across multiple separate sections of the site, leading to a mistaken view on what the issues are and how to correct them.
Now, I dont think overgeeked was being fully serious in defining the Wizard, but the fact is you have a few folks around here who absolutely are...passionately incorrect in the view of the Wizard and the 'problem' with Martials.
Yeah I mean I think there are real issues worth addressing, but…yeah some folks act like fighters can’t do anything past level 5 and the only fix is to make them Goku.
But I think the thing OP hasn’t considered is that you can bring back casters needing to be wary of combat and martials being bane to an enemy caster without making caster not fun to play for swathes of players.
Make it so casting in melee sucks more than it currently does unless you have some feature or other or make it harder to ditch disadvantage, make it so martials can potentially interrupt spells*, and let physical damage potentially break force effects and other persistent but not permanent magical effects, and you’re most of the way there. You also need to allow protection fighting and stuff like it to include movement and have passive defense benefits for the character you’re protecting, probably.
Bump what non magical skill use can do as you level up to actually meet and then exceed what real humans can do, and you’re about as close as you can get without writing a different game or killing martials for the people who already like them, IMO.
*bonus action to prepare to interrupt, reaction to do so as a reaction attack, target must make a constitution saving throw or lose the spell. Requiring setup with an on turn action, a successful reaction attack, and the target failing a save, which IMO makes it fine for it to be at-will.
Now, I wish we could get a version of D&D that is 5e except class and subclass spell lists are vastly more focused and limited and full casters are actually all interesting rather than wizards especially being “spells: the class” and nothing else. Sorcerers can have punchier metamagics and recover sorcery points more and get more from subclass, Druids can really go nuts with Wild Shape, Bard and die in a fire and rise from the ashes as an actual Bard class, etc.
The wizard core class should be much more limited in spells with every subclass granting themed bonus spells, and then make the spellbook do more, and bring signature spell and the other one down much lower level, but starting with like “1 level 1 spell can be cast at-will or bonus action instead of an action or whatever” and scale as you level. Ya know, an interesting class.
But barring that, just make martials be able to break walls of force with thier weapon (with maybe damage type mattering in some way and kick the enemy mage to stop them casting.