Look, I'm not super happy about the delay either, considering that literally yesterday I resubbed for 2 months, expecting it to be just in time to get the last couple of things in BfA that I want, do the pre-patch and play through the expansion launch, so I kinda got rekt'd, but...
I have nothing but RESPECT to Blizzard for this decision.
The reality is that the devs know better if the expansion is ready or not. We might have some idea as to how the Beta is going, but we don't have access to all of the bug reports, player feedback, QA department reports, etc. And most importantly, most of us here don't know how much time sorting all of that out would actually take. They do. And if they believe that they need more time to get the expansion to a state where it's ready to come out, I actually trust them on that.