[Galadriel55]

Luthien seemed to care very little about politics before Beren, and she only started caring about them when they interfered with her love interest. Would she really take up the burden of ruling/safeguarding Doriath? She seems quite content to live an isolated life with Beren. But if she lives to find out that Doriath is desperately in need of someone powerful to do that... hmmm... I suppose it's possible.

Saying that, though, I still think that Doriath and the other kingdoms would not hold against Morgoth. It's not even against Morgoth that they are holding, but against themselves (and once again the question comes up of how much the Curse of the Noldor is doing the evil and how much evil is done by men's decisions). The history might have looked different, Earendil might have been born to different parents several centuries later, or someone like Celegorm and Curufin might have gotten angry that they weren't chosen as the Ringbearers and the ruin of Doriath might have been even more absolute... who knows. The whole Silmarillion is about slow defeat, and even the victory at the end is not a true victory. It's the way it would be whatever would have happened.