At the end of the day Konomi is telling stories and Fuji (along with most of Seigaku) "suffers" because he was a main character in PoT and we saw his story already. All he had left in NPoT were his rivalries with Ryoma and Tezuka but like every top player has those.
Yeah, I know, but it's not like he's writing about real people or events outside his control. These are his characters and his story, which he has complete control over.
To be clear, my issue isn't Fuji's reduced page time. Less focus isn't the main problem here. For the exact same reason that I know these characters' backstories, I feel this specific character had a lot of wasted potential.
For example, I couldn't possibly complain about why Kawamura isn't in the World Cup, and I'm honestly even surprised Oishi made it. Also, I'm not a die-hard Seigaku or Fuji fan, though I obviously do regard him as a top character. Take Tezuka, for example. he was sent away and almost entirely sidelined compared to PoT, but overall I think he was treated fairly. I also think Ryoma wasn't treated fairly occasionally for the sake of storytelling or fan service (like in his match against none other than Fuji himself), but he's the main character, so Konomi always finds a way to make it up to him. And Fuji isn't the only character I feel was treated poorly, either.
That said, I do think some unnecessary characters were introduced and subplots were told that felt pretty redundant to me. But while writing this, I realized it mostly comes down to personal preference, and there are probably people including Konomi himself, who were genuinely interested in those stories.
It's a trade-off, I guess. I wouldn't do it in my own story, but I'm not the writer, and it's not my story. It was just a personal opinion, and I get it, your point is totally valid.