Amen that.Sylvanas and Nathanos are definitely some kind of social experiment by Blizz to create very personal, hateable villains. You saw it a little bit with Gul'dan (arrogant, escaped his fate once, killed Varian) but the undead pair are that idea on steroids.
Arrogant x5, abusive or manipulative to the player character, huge death counts, insult all sides nd the players were unable to lay a finger on them or deal with them properly aside from kicking them out of the Horde. They're written as so horrible and temporarily untouchable that players think they're self inserts/Blizzard stand ins for abusing the player (or in the case of Sylvanas, some people GENUINELY believe she's a girl power "SJW" character and that's why she can't be touched and is so powerful).
They are absolutely being coy about how they're dealt with in Shadowlands, which has led to doomsaying about Kerrigan or Sylvanas winning somehow, but I think it's much more likely that they are treating fighting Nathanos and Sylvanas as huge events for the playerbase. And if Nathanos can be killed by the players in the first launch raid you know that that's going to make headlines.
They didn't spend so much money on these characters bullying players and making them unhappy irl to forget them and or redeem them. They're emotionally charged punching bags and players will absolutely celebrate the chance to kill them.