Dawn is portrayed to be a lot more laid back than other characters with similar careers. She has a habit of traveling everywhere and focusing on every little thing along the way. Thus it makes sense for her to show up in the Finals in person.
Goh's departure from Ash's big finale is the explanation for why Serena, Misty, Brock and others are not there in person. They could have had Goh stick around for the Finals cheering Ash on. Maybe have Gary there as well. But the show decides that Goh has to leave despite being Ash's companion for the entire duration of JN up until then.
Pokémon is not real, but it does represent reality from time to time. That's why Ash losing in Indigo League was such a big deal back then. Because narrative-wise, you expect Ash to win as the protagonist, but he doesn't in the Indigo League. In fact, he doesn't even fight Gary who gets knocked out before Ash. They made these choices to show that reality doesn't write a hero's journey narrative for you. You have to make it work yourself.
Goh wants to be there for Ash, yet that would require him sacrificing his dreams for Project Mew. He doesn't want to leave Ash, but he also doesn't want to lose his dreams too. The same can apply to Brock, Misty, and Serena. They want to be there for Ash, but likely had other priorities to deal with.
And when you get down to it, there'll come a point where Ash and Pikachu just travels alone. Perhaps disappear for a few years and only found on top of Mt. Silver. Reunions are times to cherish, but priorities always parts them.
As for Serena being there in finals... Honestly, I did expected Serena and Lisa to do live Pokémon Performance/Contest during the half-way time to give Ash a little boost of confidence.
Firstly, the part about Dawn being the most 'laid back' is patently false, because she was shown to be the most focused and driven out of Ash's companions for the longest time; the one with the clearest path, and the most drive to succeed. Hell, May started her journey without a goal in mind, and Serena
switched career paths after her journey so she could learn more about being a better performer. The only reason she feels 'laid back' is because Journeys has portrayed her as a stagnant character who hasn't done anything of note since DP, while other characters have made visible progress.
Secondly, most of Ash's pre-Alola companions have a habit of traveling everywhere. The only ones who're slightly 'inflexible' are those who're stuck with Gyms to manage, and even
they can find time to travel halfway across the globe. Hell, Brock's studying to become a doctor and still finds time to camp out in the wilderness, as we saw in the Rocket radio episode. Plus, the bit about 'focusing on every little thing' is incredibly vague, and I don't see how that wouldn't apply to Ash's other companions.
Anyway, what I'm getting at here is that the basic reasoning here completely falls flat. I'd at least understand if
nobody showed up, but Dawn
did; making all the other absences stick out even worse than they normally would. Hell, even Ash's
mom isn't there! Pokémon battling is the greatest sport in the Pokémon universe, and Ash is just about to become the single greatest athlete in said sport; what parent
wouldn't show up to cheer on their only child in a situation like this? How does that 'represent reality'?
With Goh, at least, it's understandable because he's also in a situation where he has a once-in-a-lifetime opportunity that he absolutely
cannot miss out on, otherwise he'd 100% be there. On the other hand, Misty, Cilan and Clemont are Gym Leaders (though Clemont, with his Clembot, has even less of an excuse). Brock's studying to become a doctor. Iris was literally
there, but decided to leave for...reasons. As for Serena, she isn't even working towards her actual end-goal (Kalos Queen) right now; contests are an overglorified side-quest. The Alola gang...I dunno, for all the hype around them being 'family', you'd think at least
they would try to make it there.
Are you saying that all of these characters, on the same week, maybe even on the same
day, found life-changing opportunities or got stuck in something that is so immediate that it absolutely can't be put off? And out of them all, only
Dawn was the one with enough free time?
I dunno, man...sounds like incredibly shaky logic.