The problem with not having seen a single Mega Evolution at all yet could lean in either direction:
Either there are no new Megas whatsoever, so they don't want to show any in the pre-release materials to get peoples' hopes up. Think of how they showed small snippets of Pokémon-Amie or the character customization: they haven't officially announced either of these things but through what we've seen of the trailers, they do have new aspects to them. If there are no new aspects to Mega Evolution (aka no new Megas) there's a good chance they don't want to show it occurring before SM's release.
On the other hand, they may still be waiting to show off its return, and the reason why they're waiting for so long is because they have something major to show alongside it (aka new Megas). You also have cases such as the final starter evolutions not being shown before the games' release. Of course they will be there, but we just don't get to see them. For "important" things sometimes they do take a long time to reveal them.
So... right now things could go either way. We have another news update in a few days and we have the rest of the month, all of October, and even the beginning of November for anything to be shown. It's literally anyone's guess at this point.