Deathwatch CAN have some limiting problems:
- Female players you have MIGHT feel picked on for their seeming lack of character options
- Your favorite Chapter might not have been touched on, leaving you feeling unappreciated
- There are some things NO Space Marine is presented as being particularly good at
- Every Space Marine is a Space Marine, so some diversity is lost (less conniving, dashing, or whatever other adjective fits characters)
But when it all comes down to it, I doubt the Space Marines game will fall off so easily. 40k fans are, for the most part, Space Marines fans. They either own a SM army, or they own a ___________ Army AND a Space Marine army. They have great diversity between the "best of the best" Ultramarines, the Space Wolves, the Blood Angels, Black Templars, and on and on. As an army, they are powerful, multicapable, and don't require a HUGE investment in money, sometimes, compared to say Eldar, or $50/piece tanks IG. At least a decent portion of the player base for DW have to also be 40k TT players; no one else really should have the scope of knowledge of the 40k-verse to know what's going on, and those players, those Space Marine fanboys and fangirls will long to play as Space Marines in RPG. When I first saw Dark Heresy years ago, I asked my buddy, who owned the book, what the Space Marines in it were like, only to learn that they weren't there. I was shocked, as I didn't think they'd make a 40k RPG that wasn't the Space Marines. So yeah, DW is probably going to survive as long as the franchise of line (RT, DH, RT, OW) does. If FFG has a game set to fail, Edge of the Empire can be it; DW lacks many of that game's already apparent failings.
The only thing I wish they would do is a better meshing of the games under the 40k umbrella. Using DH as a beta for RT and DW, then changing numerous things between them, and then giving us BC, which is so different I try not to even open it, sometimes, is annoying in a game that strives not to repeat entries. They should all flow together, so that Inquisitors, Guardsmen, Space Marines, and Lord-Captains can do the same stuff, even if it is at different power scales, but with so many massive differences, at present this is not really possible.