And because of this, there is no way they will make new characters in 10.0 start two expansions back. Especially if one of the themes of 10.0 is faction neutrality, it would be crazy to start new players off with the war.
Having new people start in SL would be silly, and having people start in BFA will likely go against the themes of 10.0, especially if Tyrande spares Sylvanas which signals the symbolic “end” of AvH.
Remember the whole “new book” thing, why wouldn’t they have a start over point?
Not to mention that BFA and SL set up a new racial storyline for almost every race that can be used for new old world questlines. SL in particular sets up King Turalyon, Nelves forgiving the Horde and Voljin as a loa. Also Thralls return to shamanism (finally)