Tony Vargas
Legend
I find the discussion of core vs module a bit more constructive than arguments over what has any right to exist, at all.Supposedly, the optoins in 5e will be more at the forefront than they've ever been, so hopefully this is not true; i.e. hopefully no one will be forced into anything. But as to this point, if you have to choose between what's core and what's an addon, core should probably be about D&D's heritage and about what's popular, which is where WotC is at right now.

However, it's not /only/ our game. Obviously, Hasbro owns it and WotC is developing it, making it their IP and their baby, respectively. Business factors and the challenges of design come into it, as well. And, there's the theoretically limitless ranks of gamers who will come after us, and will, on their way to becoming jaded long-time gamers, be newbies for a while. Whether new players embrace the hobby like we did, or try it out briefly and don't, depends on their first experiences and perceptions of the game. I'd imagine that first experiences that are confusing, frustrating, and leave you feeling like the whole thing is some big in-joke that you'll never know the punch-line to, would tend to push new players into the latter group.
One way to try to manage that, of course, is a 'starter set.' Lots of us started with the basic set and moved on to AD&D, for instance. It was good enough for us, so it must be good for coming generations of potential gamers, too, right? Maybe not. When we started, there weren't decades of D&D history to get acquainted with, and the 'old guard' of established D&Ders weren't that old, it just wasn't that intimidating a hobby to jump into. Today, it is. Each existing edition is /huge/, and a new edition promises to become huge, too. The presence of a simplified starter set as much as says "the 'real' game is too much for you." Complexity (like a wealth of modular options), implicit assumptions about play style and archetypes, and built-in 'rewards for system mastery,' all make the game hostile to newcomers and paint the established community as elitist, and a 'simplified starter set' just punctuates that.
Another way to try to make the game easier on new players is to avoid all the above and make the 'core' of the game the 'face' that new players see. They start playing the same game with the same core books as everyone else is using, so there's less of a sense of trying to break into some sort of weird nerd-elite. But, they also have to start on something closer to equal footing - meaning fewer implicit assumptions about how the game must be played (just like it's always been), minimizing rewards for system mastery, and minimizing the complexity of the core system, at least.
That's a tall order, and the current design direction of 5e isn't even trying. Rather, it's making core for us old D&Ders, and throwing in 'crystal clear advice' to basically tell new players that they need to play like we have in the past. I can't imagine that going really well.
IMX, 4e, with it's common structure for all classes, relative encounter balance, and ease of DMing made for some pretty decent first experiences. I've introduced a lot of players to 4e, and new-to-gaming players pick it up very easily. Everyone, regardless of which class or role they ended up with is engaged and has a fair shot at having some fun with it. Long-time and returning D&Ders, of course, were a different story, they had definite expectations...
Ideally, then, 5e core needs to be approachable - simple (not simply bare-bones), consistent, easy to learn, and not limit archetype by style (or talent) of the player - but also familiar. Since experienced gamers inevitably want to look deeper into the rules than new ones, that familiarity could be just a bit deeper than the core rules. IMHO, the 'perfect' system to accomplish that would be one that does have a common structure for all PCs, but allows a great deal to be traded-out and customized. In that way, the game is easy to learn, but has a great deal of depth as you play more and more characters.