The playable race is Worgen, not Gilnean. People want to play Worgen when they pick Worgen, not specifically Gilnean humans.
If every single last Gilnean human was transformed into Worgen, I don’t think the majority of the Worgen playerbase would mind, because it’s not what people want out of the race.
I’m not saying it doesn’t make sense for her to lead. I’m fine with Tess being the Queen. But she is a human leading the playable race that is no longer human. The direct analog to this would be if Calia had never died and had come back to be on the ruling body of Forsaken.
In fact, I imagine the whole “light undead” thing was an attempt to have her be undead and not caused by the Scourge so she could lead the Forsaken but it wouldn’t feel weird for her to be a human still, an attempt for Blizzard to have their cake and eat it too. I’m not saying that conciliation is good, but it was a consideration that Worgen were not given.