While Demon Hunter does give ground to the idea of more Demon Hunter-like classes getting into the game (overly specific concept, heavily rooted in a popular character that doesn't draw on any specific fantasy archetype), Dark Rangers honestly have even more hurdles that Demon Hunters, I think they have even less room for expansion than Demon Hunters did, I don't really see Dark Rangers as a tanking or healing class, it just doesn't really work the base concept, at most i'd see them as a 2 spec dps class.
Theres also how "unique" a class they actually are, while Sylvanas displays several unique powers (domination magic, chain arrows, banshee form, summoning banshees, throwing shadow blades) you go to more generic Dark Rangers and Nathanos who mostly just display necromancy-flavored Hunter abilities, i'd say this isn't a huge deal though, If a Dark Ranger class is ever on the table it would almost certainly be made to resemble Sylvanas' abilities regardless of how exclusive they were to her beforehand.
Besides if character popularity was a prerequestite for new Classes we would have never gotten Monks (I wouldn't exactly call Chen Stormstout a Illidan or Arthas tier character).