That is not for us to decide. Garak could've been made into a main character instead of adding a new Dax, but the showrunners decided otherwise. Even if Adira turned out to be a wasted opportunity, that doesn't mean they had no right to be created in the first place, and especially nothing dictates that Keyla or any or the other existing secondary characters be elevated into the main cast in their place other than your own existing interest in them. There's no binding law of storytelling that requires telling more stories about them.
The whole Dax situation was a badly handled debacle that maligned the entire final season.
Adira WAS a wasted opportunity, on almost every front.
And the only true rule in storytelling is, tell a story people want to hear.
Coutts's appeal is the very definition of opinion. A lot of people do like her, though (And Owo as well)Again I ask why? What about Detmer's character and Coutt's performance has earned this? I'm not seeing this. She's a competent actress but no "spark".
What part would Detmer play in the storylines of Season three? Not Adira's part, that's for sure.
But Adira's part? Anybody could have done it. They're a plot device, a cipher. What do we know about them? Where are this 16-year-old's parents? Why were they on a generation ship? How did they end up back on Earth, in the military? How did she get into Starfleet? What's their favorite food? Hell, what's their last name?
It's maddening because the only reason Adira has a Trill symbiont is because that's what they needed for the plot. It's never even been brought up since then.
So whatever part Adira played could have been done with, literally, anyone else. Aside from the Trill story, which was a one-off, any character could fill the role she played in the Kelpian ship.