There were far more deaths caused by the Legion invasion than the ones who died in the Blood War.
Sylvanas could've easily just shocked everyone still by going to ICC as Warchief still, kick the LK's ass and break the seal into the Shadowlands and we wouldn't have any gaps. In fact, you could say the wound in Azeroth caused so much death upon impact that it made the Maw swell. BOOM bfa can be made uncanon. all characters who died are alive.