It's the only realistic chance of dethroning MS from the desktop in favor of an open alternative.
It has no chance of dethroning Windows. Zero. Zip. Nada.
Look, no one will ever be as good at being Microsoft as Microsoft is. ReactOS may be eventually be 99 44/100 % Windows compatible. It may look like Windows, feel like Windows, and act like Windows almost all the time--but it won't be Windows. And sooner or later, anyone running it will run into some instance where Windows does this but ReactOS does that. Now, when this happens (when, not if) developers will say, "That's interesting, we should fix that." But regular users will think, "Serves me right for trying to use this cheap knockoff. Guess I'll just get the real thing." And if anyone asks them about their experience with ReactOS, that's pretty what they'll say.
ReactOS is an interesting project, and I wish them the best of luck. I'm sure it will find its uses. Taking significant market share away from Microsoft isn't one of them.
The same, BTW, applies to open-source clones of other Microsoft products, which is why it's kind of dismaying that the OpenOffice family (LibreOffice, etc.) tries so hard to imitate Microsoft Office interface standards. Those aren't the only way to design office software, and there's no reason to assume they're the best, either. The more you chase the market leader, the less chance of eventually becoming the market leader you have. Try to do something different and better instead. That's about the only way any piece of software has ever broken another's market dominance, and probably the only way it ever will.