Trailers are very specifically put together, they don't just throw any random old scenes in. It's their way of communicating information to the user.

Now why bother showing this clip of a Rockruff seemingly following the trainer? They must know one of the communities biggest wishes is for the return of following Pokemon. Look at how they responded with the backlash against ORAS for not featuring trainer customization.

Now after the backlash of USUM being announced for the 3DS and GameFreak immediatley having to come in and say they're working for one on the Switch, would they risk pissing off the fanbase even further?

If Pokemon don't follow you, the only reason to have the Rockruff scene present would be the piss off players. You could say they're making the world feel more alive, but like I said, trailers are deliberately put together. We already know the world feels more alive thanks to the Wingull in the first trailer and the overworld Executor on Executor island.

The purposefully showed a Pokemon following the player with a musical note above it's head, one of the symbols used in the HG/SS feature.

Pokemon following you is in, I guarantee it. Whether it's limited solely to this event Rockruff is unkown, but that would be a pretty dick move by GF, sadly one I could see them making. Rockruff is the mascot for SM, like Pikachu was in RBY, and Pikachu was the only one who could follow in Yellow.