I'll be disappointed to see some of my favorite characters left out of marketing (particularly Wanda and Vision, who will be hurt by the lack of a publicity boost when they really need it). But I don't think it will be as bad as the Superman omission.
The big problem with leaving Superman out of Justice League promos is not that we knew he was coming back, but that without Superman, there is no Justice League. That was even the point of the movie. So they were stuck advertising a Justice League movie without a real Justice League.
The first Avengers movie, on the other hand, established those six characters as the Avengers in the eyes of the world. So if all six of them are in the promotional material, then people will accept that they're the Avengers, because they are.
Since Infinity War really wasn't an Avengers movie so much as a big crossover event - part of the point of the movie was that everyone was scattered all over the universe and there was no real team - whittling it down just to the original 6 (plus a few new recruits) might help set it apart from part 1: that was the big crossover, now here's a real Avengers movie.