The debacles surrounding 4e were mostly business & PR issues (and the unrelenting malice of the edition war, of course). WotC PR is more measured, this time around, and, though we haven't had an insider spill the beans, as yet, there's no reason to think they've over-promised to Hasbro this time. So 5e should have an easy time of it.
Even so, with all the marketing screwups, 4e showed this same level of success this early in the process. as has happened with every new rev-roll. Likewise, with every new rev-roll, WotC tries to do /something/ to boost non-core sales. With 3e it had some success with the OGL keeping things rolling With 4e, it tried to arbitrarily declare everything core. Now, with 5e, it seems they may just decline to publish a great deal beyond the core, at all. Taking the easy win and just not playing after.
As I said up-thread, I'm not just talking about initial (or even only long-term) financial success, but creative and community success which
should but doesn't always translate into financial success. As far as I can tell, 5E is showing signs of being a stronger success in the community than 4E was at the same point. A part of this, even the lion's share, is the PR and marketing screw-ups you mention, but I think it is also aspects of the game itself - that it is more palatable to a wider range of D&D players, especially the long-time people that were turned off by the "Warcrafty" qualities of 4E.
The reason I put so much emphasis on this "in-community" success is that even if D&D isn't the raging financial success WotC hopes it will be, a strong core community will keep the game alive and even thriving, if on a smaller scale. In other words, if the "core few hundred thousand" are happy, the game will be fine.
The old one happened due to controversies that made D&D seem dangerous and much darker and sexier than it really was. Rumors of 'real magic,' satanism, mind-control, suicide, etc - the same kinds of silliness that made heavy metal popular around the same time, really. Hey kids, your parents will freak if you play D&D = instant fad.
Actually, I think you have it backwards. When I talk about the "Golden Age" of D&D I'm talking late 70s to mid-80s, beginning with the publication of AD&D starting in 1977 to ~1985, when Gygax was kicked out and the satanism and "MADD" stuff was peaking. In other words, all that stuff you mention ended up ending the Golden Age, not causing it. Perhaps at first it brought D&D more attention, but it had more of a negative effect than positive.
Anyhow, couple the controversies with the rise of the
real Satan, the video game

, and the boom of the early 80s was doomed to be just a moment in time.
But I think the main reason that boom occurred was simply the newness and novelty of it. It was a fad, and it is very, very unlikely that we'll ever see the numbers rise to the legendary 20 million again. I think the best-case scenario is that we see a bump up to the 5-10 million range, which is similar to the hey-day of 3.X - perhaps a bit more if WotC can stretch the brand into movies and video games.
I suppose it's not inconceivable that D&D could go mainstream the way Marvel has - but, y'know, Stan Lee spent a lifetime making that happen, with many failures and false starts in the movie biz. Think about how much and for how long Marvel heroes were on TV, for instance. Any one of them got more exposure on Saturday morning TV than D&D did (just the one cartoon).
Yet I think the recent popularity of Marvel was almost entirely brought on by the quality and success of the movies. The first
X-Men movie showed that you could actually make a great comic book movie (other than Batman, the Keaton ones being pretty good, as well as the first couple campy-by-fun Superman movies).
Spiderman and
X-Men 2 took it up a notch further, and then the whole thing exploded with
Iron Man(despite a brief hiccup with the
Fantastic Four movies).
So I don't think it is rocket science or even requires some mastermind strategy. What it does require are good film-makers and screenwriters to take it on and make good movies. What a D&D movie would require is some nerdy but good film-maker like JJ Abramsto lead the project,
not people like Ed Greenwood, RA Salvatore or Tracy Hickman.Those guys can consult, but let the movie people make the movies (George R.R. Martin is a rare instance of someone who knows both worlds, film-making and novel writing).
A more likely point of entry would be video games. D&D has essentially spawned a whole genre or CRPGs and MMOs without any of it's own forrays into that market ever really taking off. If WotC/Hasbro would finally license D&D to the right developer at the right time, it could get another fad rolling. The TTRPG would likely be forgotten in the process, but it'd give the IP a shot.
Given that I have absolutely no interest in video games, and even actively dislike them, that sounds like my worst nightmare! But it does seem like a missed opportunity that WotC hasn't really been able to capitalize on the brand in the video game market. But again, video games and tabletop RPGs compete with each other if only for "hobby time."
I'd say it's way too early to call 5E a success yet, since we haven't even seen the MM and DMG yet... the core hasn't even fully launched yet. For folks like me, the DMG is going to be the critical book: can it bring in modules that make the game something I want to play? Still hoping yes.
This is interesting because, in a way, the DMG is completely optional for 5E, but it also very crucial, especially for the long-time fans. I know that of all the core three it is the book I'm most looking forward to but, presumably, the least necessary to actually play the game.
Beyond that, what will determine the success of the new edition is what they produce next.
Core books sell. 4E's core books sold more than any other edition at launch even with the edition war. It's what happens next that will tell the tale of 5E. I would say that specifically it's what the next group of writers choose to do with 5E that will be telling: WotC has a tradition of letting many of the people involved with the core rules design go soon afterwards, so the question is: will those next group of writers follow the original designer's vision?
Will WotC break with tradition and largely keep the core designers with them?
It will be interesting to see.
Yet, all interesting questions. Now I know this may ruffle some feathers, but one thing I'd like to see WotC do is somehow create a context in which producing new core rulebooks every 3-4 years is OK and accepted and even expected. Let the game be a living game. As much as it ruffled feathers in 2003, people overall embraced 3.5. I don't see why WotC couldn't institutionalize a revision every few years, without it completely remaking the game. So we have 5E in 2014, why not "5.2" in 2018, "5.4" in 2021, and "5.8" in 2025 before "6E: The Singularity Edition" arrives in 2030?
I jest, but the point is I think WotC could capitalize on revised versions of the core rulebooks without making all previous books incompatible.
Aside from core rulebooks, obviously the old "splat attack" approach doesn't really work on account of diminishing returns. But Paizo seems to have found a way around that by focusing on adventures, with a steady stream of setting books and a few high quality hardcover splats. But I think part of their success is that they limit the hardcovers to, hat, one per season? This makes them seem less like filler and more like quality products (although it sounds like people are already complaining of bloat).
But yeah, it should be interesting to see what WotC does after the initial roll-out.