The thing about Koharu is that she was never once stated to be a companion like the other so-called Pokegirls. This series was marketed with Ash and Go as co-protagonists, and to be blunt, half the people complaining about Go wouldn't be doing so if he were a girl with his exact same role and personality. Truthfully, the idea that Ash can travel without having the token Pokegirl (or "eyecandy", in the old words of Hidaka) isn't something many will easily accept. Early on when it was clear Journeys would be just about Ash and Go as co-protagonists, there was disappointment that Koharu wasn't in the group because they were so accustomed to the Pokegirl. At one point, there was a desperate hope that Go was a tomboy Pokegirl.
Don't get me wrong, Go is effectively a Pokemon Go shill. I agree with many people who say he really needs to battle more, but that didn't stop Serena and the SM Pokegirls from being as popular as they were—especially in the case of Serena. In fact, I've got a very simple take: I think Koharu would be a worse character if she were a main character than how she is right now. Why? Because she would be downgraded from 'unique side-character' to just being yet another Pokegirl whose goal would make her a vastly inferior Gary Oak.
The reason Go feels like he's taking up focus from Ash is because the companions were effectively irrelevant in BW, XY, and SM. Dawn was the last companion who actually had lots of focus with her contests, unlike subsequent companions who got far less focus than either Dawn or May. Go's goal can take place wherever and whenever the writers feel like (unlike Dawn needing a minimum of 5 contests plus the Grand Festival). This gives the illusion that Go is more developed than he actually is, and since its still relatively early into the series, I don't think he'll do much for a while outside catching stuff—Serena took like 40+ episodes to decide on her goal, and even May didn't decide to be a coordinator immediately.
TL;DR: Koharu doesn't need to be a main character. IMO, the fact that Koharu isn't a companion and goes to a regular school is what makes her interesting in the first place compared to most of Ash's actual companions. Its been like a decade since any companion was as involved as Go, and that the sort of companion focus I've wanted since DP, instead of just being Ash and at least 2 other bland companions.