Solo Leveling was an interesting showcase in how hard it is to carry a story with only one meaningful character. MC def had his awesome moments, and it wasn't an issue for a long time. But his growth was exponential, so towards the last 50 chapters or so, the issue started coming into stark relief quite easily.
All the other characters kept falling to bit parts / uselessness / out of the story, because the MC was the only one growing in a progression storyline. That left MC with no one to interact with that the audience could care about (and taught the audience to not care about the new flavor of the moment character being introduced, since they'd turn useless soon enough). Aside from the shadow captains, and they could barely do that. A lot of reincarnation/regression/isekai power fantasies types fall into this same trap, if they can't fill out the roster and keep some level of steadiness in it.