Also, useful or not career-wise, Cindy herself acknowledges the sacrifice if you choose not to attend and go to her party:
Yes, Cindy notes that going there would be a big thing for Ian, but like they both state, that's because writing is his passion. In other words, this aligns with what i said, that visiting the fair would be indulging in what's Ian's hobby/dream far more than a career move.
In a way it's like Wade giving up on his gaming sessions in order to spend time with Cindy because he
wants to, instead of just to appease her. Something Wade hasn't done in a long time and what poisoned their relationship. It's also why imo it makes certain sense for Ian who does pick to do his thing instead of coming to the b-day party loses his chance with Cindy: because he effectively shows her that when push comes to shove, he's just another Wade.
Your reasoning is valid but I see two issues with it. The first one is that Cindy path might happen whether you refused the opportunity of going with Holly or not (because Ian didn't receive the invitation from Holly)
This is technically true, but i think that realistically getting the invitation from Holly is pretty much guaranteed on most playthroughs -- the player can only "avoid it" if they go out of their way to avoid Holly when such actions don't gain them anything in term of alternative.
That Ian/Cindy still happens even if this aspect isn't present is something i'd chalk down to EK making a concession here to avoid complicating even further the already complicated pathing -- "you don't get to hook up with Cindy because you weren't receptive to Holly only to turn her down" would probably confound and/or piss off some of those players who did avoid Holly, even more if they've done it so they'd focus on Cindy exclusively.
The second issue I have with it is more subjective, I think Ian and Cindy relationship is more of a gradual process than an unique big moment turning point
The way i see it, it's a combination of both -- the gradual process happens during the earlier outings and interactions, but that turning point makes Cindy change the very nature how she sees Ian: from "my boyfriend's friend" through "a fun and supportive friend" to "a guy, with a penis that's hard
for me."
I don't think Cindy would really, truly register, or more importantly,
care about that latter point if by then she had Axel all over her in far more than just professional capacity. Maybe if Axel then disappointed her somehow, but this would just give us another Lena, if Ian was then to pick up the pieces.