I think D&D is too broad a market for this to be reasonably accomplished with a single product line, and attempts at multiple product lines run into the problem of resources being used on products that, while they may sell to people who wouldn't look at your main line, don't sell as well as mainline products. (If you make 8 AD&D and 4 BD&D products, for example, you may capture a broader audience by attracting people who prefer BD&D, but you may not make as many sales as you would by putting out 12 AD&D products, because you have a larger portion of that audience that will buy anything for AD&D, whether it be 8 products or 12.)
I think you are spot on, here, with your analysis. When D&D was the
only RPG it had a huge following because there were no products that did specifically what many (most?) folk wanted, but D&D was "close enough" for everybody. Now that alternatives exist (including previous incarnations of D&D!) the chances of glomming together people with disparate tastes under one compromised banner is close to nil.
In a way I wish that D&D 4E had been published as something other than D&D; D&D just has so many people wanting to claim it as their own. I don't like 4E because it's D&D - I like it because it's a great game; the best Gamist-supporting RPG I have ever seen, in fact. If it had been published as "Points of Light: Holding Back the Dark" I might still have had a great game without hordes of folk saying "Oh, the game shouldn't be like this! It was so much better when it was "Great Blobs of Shade", why can't it stay more shady..." or whatever.
This is why, as I said above, the one thing I really want from WotC (although I remain curious about the new Ravenloft game, which may not be wholly dead, and would give much for SAGA 2nd Edition

) is some way of rereleasing the backstock electronically. I think they might even see a sales spike if they do it on a 'vault' basis a la Disney's DVD releases--I know that I kept putting off a lot of PDF purchases back in the day because I assumed they'd always be there.
I never got into Saga, although I hear that it was similar to the last StarWars system, and I found that still to be too compromised to be really good for plot-driven play; maybe it would cope with explorative OK. Definitely sub-par for gamist.
This was one of the first signs that James Wyatt and I were not on the same page about what we like out of D&D. For me, first and foremost, D&D is a game about those fairy tales, about legends, about myths, about stories, and storytelling, about the power of imagination, the fear of the unknown, and the magical world we wish was there.
This is a nice example of the mix of desires out of (older) D&D. I
never found D&D good at this sort of thing; even RuneQuest was better, HârnMaster is definitely better and GURPS is pretty reasonable.
I find that, with this kind of play (explorative, basically, with many foci of exploration, although I think the "story" one is a bit of a myth), the setting is at least as important as the game system, and each setting demands a system to suit it, really. A simple core system is often best, anyway, but possibly with lots and lots of setting-based embellishments. I think maybe this is why d20 is so popular with "explorationists" - the base system, if you strip away all the tactical stuff, was very simple, and could be customised to a setting easily. The only problem with 3.x from this angle was that you were left with a few problematic "core" bits, like levels (especially the experience system) and hit points, that were hard to remove - hence why I pick HârnMaster (in Hârn) or something like Basic RolePlaying, Pendragon, Traveller or Bushido.
Sure, that involves killin' goblins once in a while (and one wants good rules for killin' goblins), but it involves so much more than that, and reducing the game down to raw monsterslaying ("You don't need rules for anything else! Whatever the DM wants!") has been the origin of so many of 4e's problems for me.
Even
with the sort of rules
I would want in 4E for non-combat challenges, I don't think it would suit you. You seem to dislike Gamist play in general - but that is exactly what I want out of 4E; it's what I think it's good at. If the moment of tactical puissance is not the focus of kudos giving, then you might as well cast much of 4E away, IMO. When I play with a different focus, then, I do exactly that - I use another system.
Which is why I'd bring the focus back out, hit it broader, make it about the entire adventure. That involves combat, but the goal is not combat. In fact, in certain adventures, combat could be a decidedly horrible idea (because the enemies are so powerful or bizarre that there's little hope of survival in direct combat).
Which would create a perfectly viable roleplaying game, I'm sure. But too compromised to spark my interest and certainly no substitute for either 4E or HM. More like a retread of older editions, which I drifted away from decades ago.
Narrative player? Awesome, you get to experience the story of the adventure.
But what if you want to
make the story, rather than simply "experience" it? There is a solid thread or Narrativist play that wants to
drive the story, not be the audience (or even a scripted actor) for it. No version of D&D has really permitted this for anyone except the DM.
Sandbox player? Awesome, you get to explore a world full of adventure.
With a good dose of disbelief suspension, this should be feasible, sure.
Game player? Awesome, you get to fiddle with numbers to make sure your party has a successful adventure, using strategy to find the most efficient solution.
Except that experience shows that systems designed for exploration (both the above types you define) is susceptible to breakage by keen gamers. 4E fixed this, just barely, but the cost of this for "dreaming" play is pretty clear.