In general, I am still convinced that BFA was originally planned to begin with the Alliance's attack on Lordaeron. This is what we were being prepared for with WOTLK. This is what Before the Storm could have preceded. And this is what Metzen was conceptualizing before he departed. Even the BFA cinematic doesn't mention Teldrassil at all (if we don't count the ones that came out later) or anything like that (and it's supposed to be one of the longest things in production).
I don't know, maybe I'm wrong, but I think that's how it happened. They needed to start a war. Everything was being prepared for the Alliance to attack Lordaeron, but how could the Alliance attack when their leader was Anduin? They couldn't ruin that character, even if he had some significant reason to attack. And so Afrasiabi, with his love of unexpected huge events, began to push his idea with Teldrassil.
Of course, there are many contradictions in all this, but I think that's roughly how it happened. Moreover, Afrasiabi was even happy to destroy Sylvanas' character.