In my opinion, the problem with BFA consists of two areas: The first area of failure is due to development time.
A game like WOW, a whole addon and all the stories cannot be rewritten easily. Especially since with the release of a game the second one is already started to be planned or even programmed, which takes about two years. So you had to change some points subtly enough that they still fit with the already developed material. Which limits the decision what you can do.
Also the cinemas take correspondingly longer, the Cinema-Team consists of 1 leader, 8 authors and about 20 renderers, so that’s it, such a team needs its time for so many cinemas of different quality and costs.
The second problem is that there was a change in the leadership and also some writers changed their positions or left the company.
Meaning even if we didn’t know exactly how it was originally intended, the plot wasn’t as you described it at the beginning, but originally it was Sylvanas who poisoned Vol’jin, also in the original phase it was thought that Sylvanas would become the bad guy and Vol’jin would survive, but stay hidden.
That Teldrassil wasn’t planned, well, bought, but I think that Lordaeron was planned at that time and that Ashenvale was in focus as well, because there was a strong focus on it, a warfront was mentioned as well. On top of that, a deliberate betrayal of the Alliance by Sylvanas was also planned at that time, because that would probably have been the spark that broke the camel’s back to MOP.
Also the book what we find was mentioned quite early in the beta and was originally part of a quest line that created a chronology in the areas, it was planned from left (Aszuna) to right )Stormheim).
And should reveal the plot of Sylvanas. But BFA was meant to be a war and in this war a united Horde would have stood against a united and angry Alliance, if you look at the descriptions of the Alliance, it was aggressive and inclined to wipe the Horde out for good.