There's a point where being modular stops being D&D and starts being GURPS.
If everything can be swapped around, there remains nothing distinctive about classes. It stops being a question of game mastery and creative development, and becomes an opportunity to maximize benefits along a small set of possibilities. Things won't be balanced, and there will be optimizable edge-cases, that will be reinforced through supplements and post-core developments. That's inevitable.
Right now, the designers seem to claim modularity as a catch-all do-it-yourself free-for-all: it saves them committing themselves and risking alienating a small group of potential buyers in exchange for pablum.
They should take a stand for what they believe will make the game solid. If they want Vancian magic, then choose that and don't provide a module that allows you to swap in at-will spells. With a module, there will be a clear choice to be made - whatever is optimum. The same choices will regularly be made. Previous editions have shown this to be the case when only some choice was available -- increasing choice only increases the speed at which edge-cases will be pursued.
I want less modularity, and something actually fixed about the world I want to play in -- some parameters and limits to foster creativity and innovation, and not a cop out.