The Forsaken and Sylvannas as primary plot devices fail before they even begin. They shouldn't have any motivations to continue on, to profligate, or "secure a future". They should die out over time. The "solution" Sylvannas seeks for the Forsaken is utter bullshit that should be shunned by the rest of the Horde; but of course the Horde is just shoehorned in for the ride because the Forsaken are members. Their entire motif in Vanilla was their miserable existence, their misery and suffering, the underdogs scraping by so they weren't tortured - why they chose to shift it 180 degrees is beyond me, and it's the reason I don't think they'll ever be able to build a compelling story using Sylvannas going forward.
We do, though. It was patriarchal, which meant Arthas was to be the next king. Calia was going to be wedded off to a random lord, which ended up being Deathwing in disguise (though the marriage never ended up happening, would have been kind of interesting if it did but Cataclysm turned Deathwing from an insane but conniving schemer into a ME INSANE KILL BURN DEATH guy).
Well, yeah, she wrote it.
But she didn't kick in the door of the developer's office and say how BfA's story was going to go. The whole team discussed and made these choices potentially before she was ever hired. It's unfair to criticise her for the direction the story is taking this soon after she was hired, if ever, no matter how much you might dislike it or her.
Blizzard have said multiple times now they're plotting the basis of expansions two in advance, which means, random changes aside, they already knew what would happen while Sylvanas was Warchief when WoD came out.