Here are 3 reasons I believe that the expansion will launch earlier than most believe.
1. Financial junk:
If they launch the game near the end of a quarter then they can brag about the sales in the reports for both quarters. Also helps by launching outside of the Nov-Dec time where most ppl buy presents that can essentially steal from the WoW hype.
2. Competitive junk
MDI and arena heck even the WF race are all in one way or another important to the game and especially important to the hype for BlizzCon. Let's say people are correct about it launching in December. That launch would place BlizzCon in the pre-patch limbo meaning no competition at BlizzCon because the game wouldn't be balanced around BFA. That means that for there to be any WoW competitions then the expansion would have to be out for a minimum of 6 weeks before BlizzCon ( 2 weeks for raid unlock another for mythic unlock and 2 more for the race with 1 for rest)
3. Support
We saw blizz state they want to wait on releasing a patch until after the holidays because they wouldn't have enough tech support for launch..... with a small patch..... imagine how it would be with an expansion.
I suspect that the latest is the 29th of September 2020 for the reasons stated above.
Alpha early Feb
Beta March
Pre-patch August