Hold on a second right there; the rest of your post seems to be written with the belief that Legion “retconned” BC in such a way that major events in the narrative are dramatically altered, and I don’t think we’re on the same page there. The example you give is a bit outlandish;
Legion’s plot is only a retcon in the sense that it introduces more nuance and context to the events of BC that weren’t known before, but it doesn’t change the outcome of BC’s events nor does it break the internal consistency of WoW’s logical rules.
For instance; everything that happened in BC still happened. Illidan still died atop the Black Temple. Vashj is dead. Kael’thas is still a traitor who sided with the Legion. Akama is now the master of the Black Temple.
The only changes that were made involve context as to what Illidan was actually doing while the player was adventuring through Outland (all of which is totally plausible considering, again, we hardly see Illidan during the BC experience) and new information regarding the physiology of demons, revealing that they can’t die unless killed within the Twisting Nether. And this is the one thing that allows Illidan to be brought back.
What makes Legion good revisionism is specifically the fact that the additions it makes to BC break neither the narrative nor the internal logic of the series. The concept of demonic immortality is something that has already been explored in Warlords of Draenor and the Diablo series. What would make Legion bad revisionism would be if they said something like “the Raid on the Black Temple never happened, or the betrayal of Kael’thas was all just a dream.”
Once you start changing the outcome of major plot points, then we can start talking about whether a retcon has gone too far. But a retcon in and of itself is not inherently a flaw in the narrative. Let us not forget that the beginning of one of the greatest works of literature began with a massive retcon;
“Now, it is a curious fact that this is not the story as Bilbo first told it to his companions. To them his account was that Gollum ha promised to give him a present, if he won the game; but when Gollum went to fetch it from his island he found the treasure was gone. A magic ring, which had been given to him long ago on his birthday.” - J. R. R. Tolkien, The Lord of the Rings.