07:13 ~ That sentiment should be expanded upon and taken to heart. In any good story the world fails to be as black and white as what the 'reader'/'gamer' is lead to believe in the first part of the story.
Sylvanas is one of Azeroth's oldest iconic characters and beloved by many fans. Does Blizzard or any writer actually think trashing an iconic character such as that is profitable? I know Kathleen Kennedy experimented with that on Luke Skywalker but given the ratings plummet of her "reimagining" project, hopefully everyone rightfully sees it as a horror story; Instead of something to emulate.
Krucial's video on Sylvanas's origin story, and it shows a Sylvanas in the current B4A version is ironically more similar to her 'Living' version before Arthas ruined everything. Turns out that actually going to hell was a bad idea on her part, so that is why she accepted the Valkyrs' deal the second time.
So what is the only good gal reason to want more Valkyr in Legion...?... Perhaps it is a desire to keep the Valkyr from being corrupted by their connection to Bolvar? Except who is Bolvar's master? The Old Gods, he is surrounded with their blood.
We already know that Ashara is the main boss in one of the raise in B4A... thus we know they enter at some point. So may be Sylvanas is relayed a few "visions" of what puppet Bolvar and his Old God masters are working on.
Why burn done Teldrassil? Well the lore strictly implies that Teldrassil's continued life is key to the Old Guy's main plan of breaking their jail cell. Everyone seems eager to forget this part of Teldrassil. And it also explains why she originally had a plan to attack Stormwind... Because when it comes to war, a pacifist does make for a weak general.
So why wouldn't Sylvanas say this to even Nathaniel? The Old Gods are everywhere and in almost everything, most especially when they can literally 'see' the shores of one of the few remaining world trees.
It would be rather ironic if the writers suggest that the Old Gods would have remained in their jail cell firmly resolute, except for the meddling ancients and planting those infernal trees... Sure the dragons flights "warded" them but every wall eventually falls down, so why would they think their wall to be impenetrable AND eternal?