Yeah, he was, and his defection from the First Order to the rebels should have had more lasting consequences than it had. The way they'd had it felt like barely a a blip. Speaking of Phasma, she's another character that felt severely underused, and I felt bad for the actress, because I remember seeing an interview with her about her role, and she was excited for what was to come for her in what was to be TLJ, saying that she would be getting a bigger role, and uh... well, I guess it's a bigger role if you count being coldly killed off with very few lines.
More than my annoyance at not creating new characters, is creating characters and not giving them anything to do
But not rigidly so. The OT had its supporting characters. While the movies were about Luke, Leia and Vader, in essence a family saga, Han, Chewie and Lando all held important roles in the trilogy. You practically can't think of SW without them. Similarly, from TFA on, they were positioning Finn and Poe be similar support roles, part of 'the gang'. But they were underdeveloped and cast aside as mere token roles.