If I had to pick out a reason why 5E won't do well, my opinion would be that it's because the core of the game isn't currently strong enough or stable enough to support a modular approach. WoTC also seems to be approaching being modular from a point of view which is different than I feel they should.
I'm not convinced that "bounded accuracy" works as advertised. There's still a very wide range of character numbers even without magic items coming into play. More specifically, I've noticed a rather large difference between the pregen characters and characters which have been put together by players. Optimization is something which always happens in D&D; there's a reason for why "power gamer" is a recognized style of play. However, I was really surprised to see such a drastic difference during the Encounters season which wrapped up at the local game store yesterday. Even at low levels (1-3,) there was a significant difference in abilities and damage values.
Another thing which surprised me during yesterday's session was that a lot of "grind" seems to be creeping back into D&D. I can't yet place my finger on exactly what caused it. Suffice to say, the final encounter had been going on for nearly an hour before we had to cut it short (and have the GM simply declare we had won) because the manager of the game store was getting ready to close. In defense of 5th Edition, I do think it may have been a fluke brought on by a lot of party members who had great utility powers (such as a mage who had prepared web several times,) but not a lot of offense (despite one of the characters being a hard hitting barbarian.) I'll also offer in defense of 5E that Murder In Balder's Gate is structured in such a way that it kind of drags along at times. Still, it somehow seemed odd, and there have been a few times when it seemed as though the options I had available to me weren't really all that meaningful; so, I resorted to just doing a regular attack with a weapon and hoping to hack through more HP or casting magic missile.
Note: Due to some players not showing up for a few sessions, I ended up switching back and forth between two characters a few times. I played both a Dwarven Fighter and an Elven Mage.
At any rate, my point is that there are still many things concerning the core of the game which don't seem quite right. Yes, they are working on that. Yes, they will (hopefully) fix those things. But, in a few cases, I ponder if what I view as being wrong is actually viewed as being wrong by the design direction. Maybe those aspects of the game work the way they are supposed to, or maybe those problems just haven't been noticed. (For what it's worth, I have filled out the surveys, but I largely feel my input made no difference.)
The other issue I mentioned in the beginning of this post was approaching modularity in a way that's different than I expected. The idea seems to be that the different modules can be added to change how the game works. A particular module might make the core game feel more like 3rd edition; meanwhile, a different module might make the core game feel more like 2nd. That seems to be the idea. I applaud that idea as far as the stated goal of uniting the fans goes. However, the problem I see is that style of modularity seems designed to change how the fundamental core of the game works. I believe that may lead to problems when groups attempt to use more than one module. I also believe, if the core game isn't very sturdy to begin with, changing how the core works could break the core foundations of the game.
Normally, what I expect from a modular system is a strong core which creates a consistent foundation. Then, what the modules do is offer more detail for a particular aspect of the core. The modules shouldn't change how the core works, but should instead build upon the sturdy foundation that a strong core provides. While this does mean the core has to be more robust than what 5E seems to be going for with the basic game, it has the benefit of allowing multiple modules to be added without breaking the game. It takes more work, but it is something that can be done. There are other game companies who have done it, and have pulled it off successfully; the idea of a modular rpg system isn't a new concept.
The worry that I have for 5E is -as I said- that the core of the game isn't solid enough to provide a game that works. (Though, hopefully, that will change as design moves forward.) Bounded accuracy isn't there yet, and, even when it is, I'm not sure that it's going to mean what people think it's going to mean. To some extent, I'm concerned that the design needs more work to be ready than what is possible within the time frame many seem to believe 5E will be released in.
It may be that everything works out. There's no way for me to know for sure because I cannot see what the finished product is. It may very well be that the finished product is light years ahead of where the playtest packets currently are, and all of the modules are awesome. It's possible, but there's no way for me to know, so I can only speak on behalf of what I see right now. Where I am right now is that I don't feel comfortable blindly buying the product before seeing it and having experience with it. In the past, I blindly purchased a lot of D&D products, but it's been a long time since I have. In fact, the last time was when I purchased the core 4E books. Today, I don't feel comfortable doing that. Part of the reason is because I was unhappy enough times in a row during the recent past to now longer feel a loyalty toward the brand. Part of the reason is because now I've been exposed to other companies and other products and have realized I like what they're doing more than I like the D&D brand; as such, WoTC is now competing with other companies when it comes to my wallet. I'm not opposed to supporting more than one game; I currently play several, but I'm at a point where saying yes to buying a D&D product is my default stance; currently, I need more convincing.
I currently do not plan to buy the core game. I may change my mind after seeing the finished product, but right now my plan is to give it a pass. I don't feel comfortable blindly purchasing it.