Most (I'd say at least 75%) of the people in America know the term "D&D."
They may know the game. They may know the devil-worship rumors. They may know the books. They may know the video games. They may even know the awful movie.
Regardless, the term means something to them.
"World of Darkness?" "GURPS?" "Palladium?" "HERO?"
People outside the RPG community don't have the faintest idea what those are, and they're the biggest competitors to D&D (but see below). Probably less than 5% of the American populace know anything about those games; most don't even know they're games at all.
D&D has uncontested brand recognition like, frankly, no other product I can think of. McDonalds? We know Burger King almost as well, and most regions have a third option just as prominent in their area, be it Wendy's or Hardee's or whatever. WalMart? Competitors K-Mart and Target still stick around in the public eye. Microsoft? How many people haven't at least heard of Linux and Apple? Playstation? For all its market dominance, Nintendo and Microsoft get a similar level of coverage.
None of D&D's competitors are known quantities to the non-gaming populace. A new player interested in RPGs is going to play D&D nine times out of ten - if not ninety-nine times out of a hundred.
Depending on how it markets its Warhammer RPG (and how much it cares about conquering a niche market), Games Workshop could conceivably take D&D's crown. Warhammer is almost certainly the third most recognizable fantasy brand (after Lord of the Rings and D&D). It has massive market penetration in the gaming community and a level of awareness outside that community no other D&D competitor can lay claim to. Money could push Warhammer to similar levels of popular awareness - one GOOD movie (or, even more effective, one good TV series) could put it ahead forever. Time and gameplay could put Warhammer ahead.
However, a funny thing happened in the '90s. D&D, perhaps the most dominant brand name in any industry (how ever niche its industry may be), teetered on the brink of disaster. White Wolf even managed to outsell TSR one year! No brand name, however dominant, can stand forever without at least putting forth a solid product. D&D, in the minds of its fans and the trickle coming new to the hobby, *wasn't* a solid product.
3e changed all that. D&D is back on top, as market-dominant as it's been since the '80s, perhaps even since the '70s. The d20 system proved marketable to existing players (bringing back the lost sheep, as it were) and perhaps even more so to new ones. The 3e core books sold in unprecedented numbers.
D&D is solid again. It may even be very good.
Solid is all it needs to be to achieve the level its publishers want, though: it is once again the default RPG, and the common ground for the vast majority of RPG players. It's again the 'gateway product' for the industry.
We've seen that the D&D brand isn't invulnerable. The actual D&D game has to have a certain appeal, or no amount of marketing can save it. Like any dominant brand, it doesn't have to be the best option for anyone, it just has to be an acceptable option for everyone. Like K-Mart to WalMart, it CAN fall if it flounders and its competitor responds - but like Nintendo to Sega in the 16-bit era, it can also resume its throne by adapting, improving, and hitting back hard.
The near-flop of 2e AD&D demonstrates just how far its publishers had drifted from their target market's desires; the unprecedented revival of 3e D&D demonstrates how adeptly its developers identified and met those desires.
A brand like Warhammer (or, even more so, Final Fantasy - if its creators ever tried the pen & paper market) could unseat D&D. But it would need a very appealing game system to go along with it - not just the equal of d20, but its clear superior.