To be fair, it was already about 300+ when I joined (the continuous debate, at least)
The thing is that no matter how many arguments I provide or of what sources they will never be enough for you. Because you see what you want to see.
The vampiric Darkfallen are extremely underrepresented in the game and their nature and abilities, as well as their personalities and aspirations, were not described well enough to make any loud statements. But we can clearly see that even in WotLK the term Sanlayn was used as a reference to the vampyric elves in service of the Lich King, while the word Darkfallen was used as their name, both in ICC and in Northrend (Borean Tundra…). That being said, the term Sanlayn can be interpreted as both the title of the organization and the convenient race indicator.
So if you affiliate with, let’s say, the Forsaken, you can no longer call yourself a member of the Sanlayn, because you can’t be a member of the two factions at once. Another question is the mind state and the physical needs of such elves, which is a whole different topic to duscuss.