I'm sure that WotC had its reasons for not officially proposing such an approach in 4e. What amuses me, however, is the difference in the reactions.
The typical reaction to "scratch out 'ranger', write 'fighter' instead" was usually along the lines of: "But I want to play a fighter, not a renamed ranger."
So far, the reaction to "scratch out 'sorcerer', write 'wizard' instead" seems to be quite positive. Words like "the best news to date", "interesting" and "groundbreaking" were used.
I simply found the contrast to be amusing. I suppose the most likely explanation is that the vastly different opinions were expressed by vastly different people.
Part of this is the "game design is 90% psychology" thing. Like with Combat Superiority basically being a series of "take -2 to damage, have this other effect" feats.
But it's not all that.
In 4e, a Fighter and a Ranger had the same ADEU + Role Mechanic structure, so the play experience was very similar no matter the label on your class. You can make a persuasive case that it doesn't matter what you call it, since it behaves basically the same. The names have no real meaning in and of themselves, they're just words for lumps of ADEU powers + a Role Mechanic.
In 5e, a Wizard and a Sorcerer have a different play experience (or at least they SHOULD). Independent of whether you use spell points or spell slots, a Wizard's schtick is to prepare in advance the magic she will use from a vast library of potential tools, while a sorcerer's is to pick in the moment from a limited list that they can use repeatedly. Wizards pick the right tool for the right circumstance, while to a sorcerer, well, they have a hammer, and every problem is a potential nail.
So in 5e, if you crossed out "Wizard" and wrote "Sorcerer," the experience wouldn't be interchangeable. You'd still be preparing magic from a vast list, not calling it spontaneously from a small pool. Regardless of spell points or spell slots, the behavior is different for each class.
Class, Spell, and Magic Mechanics
It's a little unclear how this possibly intersects with the class list, or the spell list. It's a little unclear how this is going to shake out. But there's a few ways it could.
If you want to keep the classes, and the spell names (so that you still have "Sorcerers," "Wizards," "Magic Missile," and "Fireball"), you make the Magic Mechanic a purely mechanical expression of how the magic happens.
So you have a Warlock. And via her class, she has pact magic and can access the Fireball spell (she's got a pact with some efreet lord in the City of Brass perhaps!). And her choice of Magic Mechanics only affects things like the recharge rate, power level, and preferred tracking mechanism of the spell. So maybe she is a Vancian warlock, where she spends Spell Slots to cast Fireball. The warlock also has some unique class mechanics, like Eldritch Strike, that are kind of just part of the warlock bag.
And you have a Wizard. And via his class, he has scholarly magic and can access the Fireball spell (he found it in a dusty tome of some mad evoker). And his choice of Magic Mechanics means that maybe he is a Spell Point wizard, so he prepares his spells in the morning, and then spends Spell Points to cast magic from this custom list. And the wizard also has some unique class mechanics, like a Spellbook for learning and swapping out magic, that are just kind of part of the Wizard bag.
And then you have a Sorcerer. Via his class, he has heritage magic, and can access the Fireball spell (red dragon ancestry y'know!). And his choice of Magic Mechanics means that maybe he's a Reserve spellcaster, so he can use a little version of Fireball at-will and then blow up with a big Fireball all at once (but then loose the little fireball). And the Sorcerer also has some unique class mechanics, like a Bloodline Bonus that lets them use that high Charisma to influence creatures related to their bloodline (our sorcerer can win staring contests with Dragons!) that are just kind of part of the Sorcerer bag.
You've also got the Cleric. Via her class, she has divine magic, and can access the Fireball spell (it pays to worship Pele!). And her choice of Magic Mechanics means maybe she's a Wild Spellcaster, so she needs to make a d20 roll and can cast anything from a puff of flame to a raging torrent of fire. And the Cleric has some unique class mechanics, like Cure Wounds, that are just part of the Cleric bag.
Which means that you don't have to define a class by (a) a list of powers/spells, (b) a method of casting them, or (c) a particular requisite magic mechanic. You can define them by what they are in the story, and then let the players pick out their own preferred mechanics.
Conceivably, fighters and other martial classes could benefit from this as well. There's no conceptual reason why Expertise can't be a sort of universal potential martial mechanic, accessible by barbarians and rangers and rogues who want to have combat expertise, and why other things (like an Opportunist mechanic that lets a fighter use Dailies and Encounter powers or something) couldn't be rolled up in their place. Hypothetically, you could even cross streams and have a magical version of Expertise or a martial Vancian system.
And then for the newbies, you'd have a default mode. Fighters get Expertise dice, Warlocks are Infinite Spellcasters, Wizards are Vancian, or whatever. But you're not tethered to that.
And for us wonks, you could swap out the mechanics, independent of class.
It's an interesting idea, to silo things like that. It might not be the way they're actually going, there's plenty of other options, but it's got me thinkin'...
Spell lists? - doesn't sound like it.
