The real problem I see Next having is that it's going to be very hard to cover new ground within the same basic framework as the older editions. I like the ideas behind it, but I just don't see anyone, let alone a creative team and company that has clearly struggled to focus on and implement anything at all the last few years, pulling off a game that manages to fit in between all of the older stuff without coming off as being a watered down version of all of them. It's not going to help that the brand is in trouble; not dead, but definitely in trouble. When you've lost most of the player base you previously had to earlier editions or spinoffs of those editions, the last edition is widely viewed as either a failure or at best a limited success, and all the grand goals of expanding into other genres and markets has yielded a grand total of one video game that has received mixed reviews and a continued reliance a three decade old character to sustain the novels, it's fairly clear that the core market for new products in the brand is not what it used to be, and sustaining the interest of the new blood is going to be extremely difficult without a solid and substantial core group. That lack of excitement (which is not the same as lack of attention) is going to be a major problem; a lot of people may be paying attention to it, but I don't see a lot of people clamoring for more information or being particularly passionate about rushing out to buy it immediately. From what I've seen, the general reaction is that it's going to be a decent game that few would argue against playing, but just as few would actively argue for it to be the game of choice over something else. In this market, that is going to kill it quick if they don't manage to generate a lot more excitement from the core gaming community. I don't care how much excitement they generate in the general public; without a lot of core gamers working hard to push and sustain it, the new folks will just move on to the next big thing when they get bored with Next.
If it fails to sustain itself after the initial burst, or even appears to fail, the brand will be in serious trouble. Drizzt can't carry the brand forever, and bringing back the old cast will probably simply act to hasten the end of the line for him unless Salvatore uses the unexpected break created by the 4E to continue to reinvigorate not only Drizzt and his individual companions, but the entire group dynamic; otherwise, it'll quickly become more of what everyone has already read multiple times. They may or may not be able to make the movie that they want to, and that movie may or may not be the success they absolutely need it to be, and without the core market to serve as a starting point, the probability of success in that arena shrinks even more. The general public will likely lose most of it's interest in the brand as an active product with two widely perceived failures in a row. On the flip side, if WotC actually pulls it off and Next manages to sustain itself not only as a role playing game but as a brand that truly encapsulates the whole sword and sorcery type of fantasy, then it has a chance to reach a higher level of success than what 3.x pulled off. The challenge is that it's going to take a lot of work, luck, and long term support from both WotC and the gaming community to pull off, and I just don't see the level of excitement (again, not the same as the not particularly unsurprising level of attention) to make me think it's very likely to achieve that.
I personally see it doing well enough that most of the gaming community ends up seeing it as a solid, but not particularly unique or attention getting, game system that if was being published by anybody else or under any other name, would be a great success, but because of the brand and the company, it's going to come off as more of a dud than not when it fails to meet probably unrealistic goals and expectations, and WotC will simply lose interest in doing anything more with the brand than keeping it on life support as long as they can to get as much money as they can from it while spending next to nothing on it. They'll be more than happy to let Salvatore write Drizzt novels as long he wants to, and be equally supportive of Greenwood writing more novels, but probably won't invest much in brand new characters or authors unless they have a successful movie to tie it into. The role playing game will simply fade away, as those who were already playing one or more of the existing versions will continue to play what they already have, but there will be very little effort to actively support any of the existing versions or bring in new players. The boardgames will do the same, limping along in a crowded market, drawing some attention due to the brand, but mostly slowly fading away as the competition makes more noise. I could end up being wrong, but it's going to take a lot of effort and resources to make anything else happen, and I'm not convinced that WotC is willing to throw those kind of resources at it over the long term, nor am I convinced that they are going to generate the necessary size and strength of a core player base needed to give them reason to even consider it. There will be no 6th edition any time soon, regardless of how Next does; either Next does well enough there doesn't need to be another edition anytime soon, because the brand will no longer be reliant on a core ruleset, having managed to finally successfully branch out to other markets on a permanent basis, or the brand simply gets puts on a shelf to collect dust for a while.