The issue is that Vancian magic is a sore spot for a lot of people. We don't necessarily want alternatives so much as the ability to run a complete game of D&D while removing it entirely. To put it more specifically, I want to be able to play any character concept I can think of without having to use Vancian magic which I hate, and after seeing the Sorcerer and Warlock refluffing those is most certainly not a good alternative. When I DM, I want to be able to run the game while being able to completely remove Vancian magic without gimping the game, and banning the Wizard class/archetype to me gimps the game. The Wizard archetype is so central to D&D that removing/ignoring the Wizard results in an incomplete D&D.
This isn't the only place modularity fails:
1. DM fiat/Mother May I--I don't really see how this can be modded after the fact in any satisfactory matter.
2. Resource attrition--the current game is being designed around resource attrition, which is great for dungeon crawls and sandbox and considerably less great for anything else. Like
#1 , I don't see how this can be modded after the fact in any satisfactory matter.
3. Tactical Depth--Tactically, this game is fight or run away. It lacks the deep and emergent tactics of 4E, or even the rock/paper/scissors Rocket Tag of 3E. Simply adding a grid and 3E style universal maneuvers does nothing to change this, and again modularity isn't likely to be able to fix this for people who want a deeper tactical experience than what pre-3E provided.
4. Interesting mechanics--The mechanics of 5E are very simple and not very compelling, compared to 3E and 4E. The Sorcerer comes close, but is far more interesting in concept than in practice. Mechanics that are interesting in practice are missing from 5E right now, and I'm not seeing where a modular add-on can change that.
Personally, I don't want to play a D&D based around Vancian magic or Mother May I, isn't balanced unless you play in an attrition based story, and lacks tactical depth and compelling mechanics. 5E's core as it is currently design fails at all of those things, and fails harder at them than a couple of tweaks would be able to fix. The problem with a strong core and minor modularity is that it all depends on the core, and if you hate the core modularity can't fix it. For 5E, I would be need to be able to change and deviate from the core to a stronger degree than is currently being described as possible.