Neither Daggerheart, nor any other game system before or since, needs to "dethrone" D&D to be successful, or relevant, or matter. This is not how most small companies measure their success, let alone their own viability. They leave that to the uneducated masses of online communities and personas who thrive on such inconsequential speculation using overly sensationalist tag lines to continually attract a thristy audience looking for substance. And here we are.
Personally, I am fine with (current) D&D continuing to be a household brand name. It should be evident by now that quality doesn't equate to anything when the market is dominated by a majority that consistently shows a strong preference for mediocrity, popularity, and familiarity. Hem and haw all you want, but anyone who believes D&D is where it is now because it is a great system is lying to someone; either us or themselves. That's not to say it isn't a good (or even great) game system, but the design of the game itself is not its real strength. It's just simple enough, complex enough, appealing enough, accessible enough, and popular enough to gain (and keep) the largest audience.
For many people, it's still good enough. It is simply convenient because it's easier to find people who feel it is enough, or even more than enough. More importantly, it is harder to find people who are interested in anything else because what they have is enough. And who can blame them? If every other game traces it's roots and origins to the original concept of D&D and offers nothing different, why bother if only to compare it with the game you already know how to play and can find dozens more people to play with?
Now Daggerheart has gotten my interest, which is saying a lot. I thought I was pretty much done with new RPGs. I am certainly over this current iteration of D&D, as well as more complex systems requiring more free shelf space than I am willing to afford again. I am still looking over the materials, reading the discussions, and watching some videos to gain some better insight. I don't see a system trying to emulate existing game mechanics or designs that reinvent the wheel, but a system designed to enhance what really matters most: the play experience itself. It is most definitely a game enjoyed better in a live group that can interact on a personable level that a VTT or online chat cannot fully immerse.
Can Daggerheart usurp D&D? Impossible, but not improbable. But it can offer something that many people who have had enough of thinking D&D will always be enough because no one had tried to give them something different. Maybe this system can peel away some of those looking for more or different, and it may be just enough to give the majority a new option: another kind of game with more players willing and eager to do something different.