That's what Pokemon used to do (over half the megas in XY, like 80% of the dexes in XY & SM, all the megas in ORAS, I think like 80% of the SM regionals, all the new USUM Pokemon), but very, like, provably doesn't any more?Ngl I fully expect them to reveal all new megas over time until the release.
That's just stuff japanese companies do and Pokemon has not been a exception.
SWSH only revealed a fraction of its Pokemon, Regionals and GMaxes. Same with SV. L:A barely had anything to reveal but still didn't reveal everything.
There will definitely be more Megas shown off, but I would be pretty surprised if it was more than...mmm...well hey we can just look at L:A a bit since there'll probably be about as many designs between new mons & regionals
Of the 7 Pokemon, 16 regionals and 2 Origins they revealed 3 Pokemon and 5 regionals. I also thought for sure they teased the 3 starter final forms in the classic silhouettes, but I didn't see that in my double check of the serebii archives...I'm pretty sure they did though, and didn't reveal them until after release, so I'll just assume I overlooked a news post.
So 8/28 + 3 teases. Definitely not indicative of them deciding to reveal every Mega over the next 2 and a half months.