Ignore the difference in classes for a moment. What does a "simple" wizard or cleric look like?
A simple wizard: An elementalist. They have easy control over one element or one force and can do more or less whatever the hell they want with it but lack flexibility. They'd get e.g.
Affect Normal Fires and a firebolt as cantrips, and have a few other spells like summoning a fire elemental from an existing fire, and fireball. But only a few. A simple cleric: A Warpriest. They heal, they occasionally throw down a spell like zone of truth. But mostly they hit things and rely on insight.
What does a "simple" rogue do?
1: Turn invisible.
2: Pick locks.
3: Pick pockets.
4: Disarm traps.
5: Stab people.
6: Confuse people.
Alternatively they pick one or two things and specialise in them.
How are the complex "hard mode" ones meaningfully different?
Less power, more options. Affect normal fires for a wizard certainly wouldn't be At Will.
Take the wizard for an example. The easiest way to build an "easy mode" wizard is to use the 3.5 sorcerer model: small pool of spells, lots of chances to use it.
That's the worst of both worlds.
You'd get a wizard that's fairly simple to play, but realistically is he equal to the complex mage who can change shapes, conjure fiends, and fool enemies into believing a door exists where it shouldn't?
Of course not. This is because a specialist should be significantly better at the things they specialise in than a generalist. If the fire specialist got 50% more damage from fire spells (and evocations were worth casting anyway) would they be as strong as the generalist? Possibly. But versatility is a strength. Taking away versatility and giving
nothing back is hamstringing a class. To point out a class you have hamstrung is hamstrung is obvious.
Let's try a different simple wizard. One who can only cast spells of the fire type - but counts all fire spells as if they were one spell level lower. So he can cast Burning Hands and Affect Normal Fires At Will. Scorching Ray as a first level spell. Fireball as a second level spell. Wall of fire as a third level spell. And so on. Realistically is he equal to the complex mage who can change shape, conjure fiends, and fool enemies into believing a door exists where it shouldn't? I don't think so. But if it's a thunderdome situation, our fire mage is going to win. And if it's taking on an army, the ability to fling fireballs at third level and walls of fire at fifth is
really going to help. Is this discount on spells enough? Probably not by 11th level. But I think the specialist will be highly effective until about 7th level - rather than (as for the sorceror) looking silly by
third level.
In easy mode, the wizard is crippled by his small selection of mostly attack spells
In easy mode the wizard isn't crippled by that. He's crippled by having a small selection of spells
at which he is no better than our generalist wizard. If he really got a boost in his specialist area he might be able to keep up (until the 3.5 wizard became ludicrous).
Again, the fighter who can do nothing but hit suffers against the more versatile complex fighter, who can STILL go "all damage" if he needs to as well.
And there you have the problem. The fighter who can do nothing but hit
must be better at hitting than the complex fighter. Your solution is implicit in your problem.
Personally, I can't see having "easy" or "hard" modes that can work together.
It works in 4e. You have the weaponmaster fighter (complex and tactical) and the Slayer with a two handed weapon who just hits stuff incredibly hard. About half as hard again as a weaponmaster. Which is more useful? Pass. Both have their place.
You really can't have "easy" and "hard" fighters expected to fill the same role.
Again, you've just described the solution. A specialist
does not fill the same role as a generalist. A generalist should be a jack of all trades
and master of none. (This is where the 3.X wizard fails utterly - they are masters of all trades eventually).
Either the game wants to be complex, rich, and option-full (but hard to get into) or it wants to be light, simple, and quick (but not very detailed/fiddily).
Who says?
But like basic and advanced, you shouldn't expect you can drop a B/X fighter (d8 HD and all) into your AD&D game and run as is and expect to fill the role of an AD&D fighter with weapon specialization and exceptional strength.
Yet you can drop a 4e slayer straight into a 4e game. They are strikers not defenders but can certainly fill the role of a 4e PC. And a 4e slayer is almost as simple as a B/X fighter - and certainly simpler than a 2e Weapon Specialist with Non Weapon Proficiencies - or a 3e fighter.
And a single counterexample is enough to disprove your case.