Well, this has turned into quite the thread. I obviously can't comment on every post, but let me make a few points:
2. I'm not worried about 4E failing. If this thing had crashed and burned on release, that would already be apparent. Clearly it has had some success. The questions are how much, how sustainable is it, and can it grow the hobby beyond that of the third edition era?
This is an interesting question, so we'll take it apart:
1. How Much--Hard to say at this point, aside from the fact that 4E is the center of attention and getting nearly all the press. Love it or leave it, it is casting a large shadow.
2. Sustainable--I don't see 4E sustaining itself as being an issue. It is theoretically possible for 4E to lack the success of 3E, but I don't think theres any danger of it not being able to sustain itself.
3. Growing the hobby--This one's the real trick. This thought implies that its D&D's responsibility to help grow the hobby, as opposed to serving itself and the D&D community. The OGL movement created by 3E involved D&D taking on the mantle of "being the leader" and trying to build a new future for RPGdom. In this regard, I think 4E will not live up to the standard set by 3E. 4E will not inspire the creative burst that accompanied 3E, and 4E will not directly grow the hobby beyond merely being a good game and being worthy of the title. I don't think 4E was designed or intended to do such things, which 3E was designed to do. The masters of D&D decided that a change was in order, and that it was more important to take care of D&D than to lead the industry. I don't think 3E necessarily succeeded in its lofty goals. While the OGL spawned a new industry in the RPG world, it didn't grow the hobby as much as supplant non-d20 systems and realign the RPG world around D&D, the d20 system, and the OGL. Many new books and ideas became available, but how much of this was growth and how much of it was simply increasing the options for existing gamers? Many doomsayers have been saying the hobby was shrinking since before 4E was released. I can understand Mr. Pramas' concern, as he is one of the highlights of the OGL movement, and D&D certainly is not taking the leadership role in supporting the hobby and the OGL like it has in the past. On a final note, I mentioned before about D&D deciding to take care of its own house as opposed to supporting the industry. My thoughts on this would be that the OGL and all its myriad creations created a dilution of the fanbase not unlike what happened with 2E AD&D and the millions of settings TSR published for it. While this splintering of the brand didn't occur in house and involve squandering resources like it did during 2E, the explosion of D&D/d20/OGL into every nook and cranny weakened the brand name by blurring or destroying the line between what is and isn't D&D to the point where it was difficult to determine the difference, and built up some unrealistic expectations among D&D/d20/OGL about what D&D was supposed to be.
4. My sources were talking about 4E as a whole selling in well and then slowing down quickly. In addition to that commentary, one of them noted that new 4E supplements were selling at about the same level as latter day 3E supplements. The expectation was that the new products for a new edition would sell a lot better than books released 7-8 years into third edition.
One of the complaints leveled against 4E is that the new edition was introduced to soon, while the previous edition was still popular and very much alive. As the game was popular and very much alive(which I believe was true), I would expect the sales of 3E towards the end to be respectable, and merely matching those sales not that much of an indictment. The Core books always outsell the supplements. Have RPG supplements for major games ever sold as well as the core books? Its been a while since we've had fresh new core books, and supplements are going to look bad compared to them. 4E is releasing books at a slower, more careful pace, and it doesn't have a back library of supplements that newly introduced players will want(like the Complete series, Spell Compendium, Psionics, ect.). Including that back library of titles, which were still being bought up to and after the release of 4E is going to color the interpretation of things. In addition, 4E is taking the risky and ambitious step of writing books solely for DMs or Players. 4E's emphasis on balance has prompted many DM's to allow players to use rules from supplements like Martial Power and the Forgotten Realms Players Guide without the DM owning or even having read those books, which would have been unthinkable to a responsible DM during 3E. The lack of player information in books like the FRCG and Draconomicon(and future supplements like Dungeon Delve and Open Grave) means that players can skip these books. Writing books for players or DMs only is going to affect the sales of those books, though the idea is to better serve the game by doing so. If I were to speculate, I'd say that the sales of supplements for 4E vary wildly. I'd imagine that the adventures, Draconomicon, FRCG, and Manual of Planes sell at a modest rate while the Forgotten Realms Player's Guide, Martial Power, and especially Adventurer's Vault sell very well. The big test for 4E supplements will be seing how well PHB2 sells.