Calia is not really Sylvanas 2.0. And I don't think you get the point that actual Forsaken Fans and Players have, when they are dissatisfied and angry at Calia randomly becoming the leader of the Forsaken.
First, Calia has a too recent Alliance Story. I mean, sure, Sylvanas was technically an Alliance Character too once, together with the Forsaken when they were alive. The difference is, this was back in WC3, when the factions didn't matter to begin with, because the narrative was more focussed on telling the stories of different races. In World of Warcraft History, Sylvanas and the Forsaken were always Horde alligned. Nowadays, they are an iconic, essential and traditional part of the Horde roster.
Second, your arguments make no sense. Sylvanas is deeply connected towards the origins of Forsaken Story. The original Forsaken were renegade undeads of the Scourge who broke themselves free and grouped together under Sylvanas to fight off the Scourge and the remnants of Humanity from Lordaeron. Sylvanas personally freed many Forsaken and Dark Rangers as well as Banshees were always a traditional iconic part of the Forsaken.
Third, Style. Calia just stylistically doesn't fit into the Forsaken. She has a clearly Alliance human style, as a light alligned Priest who has a rather bright color scheme. The style of the Forsaken was always defined by dark colors, horror or gothicesque architecture, skulls and blight. They are stylistically dark and, yeah, rather gothic and horror-inspired in terms of their unique racial style. Calia introduces basically an element of Disney what is supposed to be more of an original Tim Burton setting, which simply doesn't fit in. It would be exactly the same, as if we would introduce a dark and shadowy gothic inspired racial leader into the Humans or Blood Elves, who were always defined by a rather bright color scheme and style. I mean, Blizzard seems to understand this themselves if we look at Anduins new style, which represents the Human Fantasy perfectly. It is Paladin-esque, bright and knightly with a lion motive, fitting to the traditional human style which was defined by light, bright medieval and knightly elements and a lion iconography. Calia inside the Forsaken would be like introducing a knightly Troll or Orc or a tribalistic Draenei as Racial Leaders. It doesn't fit and breaks with the fantasy one has with a certain race.
I mean, they could introduce a more morally good Forsaken as the faction leader, but Calia just breaks with what defined the Forsaken in terms of style, tone and story for all of their history too much. Which brings me to the question why she has to be an light-alligned Undead and Holy Priest to begin with. The Priesthood of the Forsaken were traditionally shadow-alligned, just look at the titles of their clerics. It just rubs me the wrong way that both major undead priests, Faol and Calia, have to be light-alligned. Void and Shadow are themes, which already lack representation inside the playable races of World of Warcraft, the only ones really representing this aspect being the Void Elves. With the cult of the forgotten Shadow re-introduced and Blizzard seemingly wanting to make both the Light and the Void more morally grey, I don't get why Calia couldn't have turned into a normal Undead and chosing to study the teachings of the forgotten Shadow, so that she can understand her peopl better who know seem to be practicing a void-based religion primarily instead. This way, Calia could have received a unique model which represents the common style of the Forsaken better, she would have a deeper connection to her people, because she converted to their new religion instead of sticking to the more human-centered religion of the light which many Forsaken left behind and she would have served as a good representation of a Shadow Priest, offering a counter balance to prominent light priests like Velen and Anduin.
To this day, we don't have a single prominent Shadow Priest in World of Warcraft and not a single void-alligned character in the Horde, despite the fact that traditionally in World of Warcraft, the Horde was closer alligned to the Shadow, with both Forsaken and Trolls having more shadow-alligned racial priest spells. The light was always more of a traditional theme of the Alliance. And while I guess we have to live with Void Elves being a part of the alliance right now, focussing more on the forgotten Shadow would have been a good opportunity to bring in the void into the Horde, who are themselves currently overly alligned with the Light, with Blood Elves being now stronger alligned with the Light than with the Arcane.