It's just not what people wanted from Star Guardians, no matter how good they may be. People want characters in a theme because THEY WANT THAT SPECIFIC THEME. They don't want those characters added into a theme and then Riot go "OOH, LOOK! WE MADE THEM DARK AND EDGY AND TORMENTED BECAUSE WE WANTED TO TWIST THE THEME." That's not what the theme is, and it's disappointing when Riot does this shit. It's like going to a burger restaurant for a medium-rare burger with cheese, only to be told that they're a vegan restaurant now and you have to order some quinoa burger with fake cheese instead.
Furthermore, it all circles back around to Riot terrible storytelling tropes. They seem unable to tell a story without resorting to just throwing all of the cheerful characters in to dark places and smearing them with horrible tragedy. It's happened repeatedly, and it gets super old. It's stale, it's boring, and they crutch on it really hard. They could have made dedicated villains instead, like Queen Beryl and her menagerie of henchmen, but no, they decided to go the same route they went with just about every other bit of storytelling they've done, including the Darkin, Odyssey, Cosmic/Dark Star, Project, and High Noon.
A thing you're missing (and a lot of people for that regard) is that it's possible to do something that's beautiful and completely be off the mark in terms of the theme. The theme here was "magical girl anime skin," and they went "Dark Star Odyssey anime skin." It's like if your art class was tasked with making a sculpture as an assignment. But you turn in the most beautifully done oil painting of a rose. You fail the assignment though, no matter how beautiful your painting was, because you didn't submit what you were expected to submit.
