It was not "by September 2018", it was "by 21st September" which means Summer 2018. You have no evidence someone pushed BfA to August 13/14, imo more likely it was planned, but later Blizz refused to delay it (member that BfA exact date was announced reeealy early, something like 4 months before launch?). SL showed us that even month delay can fix ton of bugs.
Also I disagree that "wait" in 8.1 was problem. First: what wait (for raid)? Uldir had just 3 months, extremely short for full raid. Second: I love weekly questlines leading into raid, slowly rising hype.
Sadly just Nighthold, Argus and BoD had that - and only Nighthold was almost flawless (first big zone, then 9 chapters of weekly story questlines, only problem was lack of Gul'dan).