Well, I agree that these characters should play a role, but we should take into account a few things.
Lately, the manga has been focused mainly on Boruto and Tentou. Even Sarada and Mitsuki have been somewhat sidelined in recent chapters, and we haven't seen team Moegi (ino-shika-cho) or Metal Lee since the chunin exams. If even these characters can just be sidelined and not shown, then it is certain that characters who are currently anime only can be thrown to the way side (not that I want that to happen).
Additionally, the Naruto series threw side characters to the way side all the time. Team Guy. Team Kurenai. Etc... I'm not saying that this is a good thing, but it happened nonetheless. Who's to say that this just won't be the case with Sumire/Denki/Iwabe?
There is hope though. Even if the anime stays true to the manga, the manga is only in its first arc of new content. Like I said before, right now the manga is just focused on Boruto and Tentou, but once this Mujina bandits/Shojoji stuff is done, the manga may move into a new arc that is more inclusive of other characters. At that point, Sumire/Denki/Iwabe may make their manga debut.