This CoDzilla thing is a phenomenon created by 3e. Take all of the stuff that was added to these classes away and return them to their original states and the problem goes away.
No it doesn't.
All of the things that restricted spell casters in the past were discarded as unnecessary when they were designing 3e. The classes all got boosts of power. The problem was these boosts did little good for the non casters and opened the flood gates of abuses aplenty for the spell casters.
It made it a lot easier and simpler, and created an even more huge gap, but the gap itself was always there.
I think the more potentially more powerful classes need to be reigned in more by making them harder and less desirable to play. Who wants to play a wizard with only 10 spells? Who would want to try to be the mage if they had the real chance of failing to learn the really good spells? Who wants to play a character who is likely to lose half of their castings in an encounter to minor damage? Let them go nova, then make them continue the adventure using mundane equipment like everyone else.
Except it rarely, if ever, worked that way. Nor is it even a desirable way for it to work. If you're either awesome or you suck then you'll suck when it doesn't matter and be awesome when it does. If you suck always, then the class just sucks. If you can get around the suckage, then you will, which is exactly what happened in AD&D. I can count on one hand the number of times one of my wizards lost a spell to an attack in 20 years of playing AD&D. It just wasn't an issue. Sure, NO character is 100% capable every second, but casters were vastly more capable than non-casters, even in OD&D.
Clerics can fight and wear armor why give them damaging attack spells too. Clerics were regarded as the class of least desirable because all they could really do is heal. Lets go back to that. take all of the overpowering buffs and metamagic feats and dump them into the trash.
Well, I'm not saying those spells shouldn't exist, but clerics IMHO were boring because the party REQUIRED them to cast nothing but heals. The rules were structured that way, you couldn't really avoid it. They had many other potentially interesting spells, but it was always a bad idea to take them. We'd just be right back to the "nobody wants to play it" AD&D cleric. That's not where I want to be.
I also think the competition between players at the table and the seeming envy of casters by those playing non casters might be an issue too. D&D is a team sport play as a team.
Yes, it is a team game. So why would you want some members of the team to vastly outperform others? It isn't about 'competition', it is about some of the people at the table feeling like their characters are useless most of the time over a large percentage of the game, or that they're at best secondarily useful and their team would be better off with another caster. It makes no sense to me.