This is an interesting topic.
Will DND 5e be the Number one Seller in Tabletop Roleplaying Books?
Hard to tell/say. The market is quite different now.
There is a lot of High quality products out right now they are competing with other then Pathfinder. Numenra and 13th Age to name a couple.
That's nothing new. D&D was competing with higher quality products almost as soon as other publishers jumped into the market. Even after the fad waned, D&D was the only RPG with mainstream name recognition, and the one RPG most gamers started with, giving it tremendous longevity.
Add Kickstarters to the equation and you have one broke gamer.
That is, indeed, a difference. Even so, it's not that different from the advent of small press publishing in the 90s, which, yeah, hurt D&D a little, but arguably not as much as CCGs.
The real question is. Is selling/producing the most books the most important and crucial thing to DND5e (And quite frankly DND in general) success?
I'd say no.
It's the only thing that's made D&D successful in the past. There's always been some speculation that it could go mainstream with a board game or card game or movie or video game or novels or comic books or whatever - it's never quite panned out. In 2008, they pinned their hopes on a massive subscription-model revenue stream over and above just selling books, and it didn't work out so well. In the 90s, TSR drove itself out of business by betting to heavily on publishing novels and putting out failed collectible games (when CCGs were the latest fad).
DND IS the name brand. Marketing and Licencing the brand is what is going to make DND5e successful. Movies, Video Games, Novels, Toys, T-Shirts. Getting attention of the General Public.
We've heard that tune many times before. Admittedly, it'd be nice to see someone finally dancing to it, but I see little reason to hope.
5e is an easy to learn and simple system perfect for this audience. I think with this route, WOTC will have great success.
Least of all this reason. 5e is /not/, even in it's Basic form, a simple or easy-to-learn system for the new-to-RPGs player. For experienced players, even for returning one-time players of long ago, sure, /because it's familiar/. Even a player who started with 4e is going to have a fairly easy time assimilating the archaisms of 5e. But or the new player, peeping in from the mainstream with, maybe, exposure to MMOs or a CRPG or something as foundation, 5e is as arcane as 3.x - and only barely saved from being as counter-intuitive as AD&D by sticking with d20 core mechanics. For every improvement in clarity/consistency that makes the game simpler or easier to understand (like 'vantage) there's several steps back (like Vancian, saving throws, inconsistent class advancement), not to mention unchallenged D&Disms like armor making you 'miss,' that have always made new players scratch their heads a bit.
It's not easy to learn or simple, it's just familiar and not all out yet.