I've long believed that Serena was part of an option to write Ash out at the end of XY, that ultimately wasn't taken. Without settling down with Ash, she's a character who found a path in performing during her series, growing both in that venture and personally. I agree that her character is unfulfilled at least in part, but the roadblock is in Ash's storyline, not in hers.
Ash is the main character of Journeys (and the anime), Iris isn't. For long-time fans it's development in her story and a nod to the BW2 games, but for casual viewers, she's just another random character of the day, introduced to be x person who has x role and does x thing. Iris could obviously become champion on her own, and her becoming champion doesn't impact Ash's story at all; they still barely intersect. Serena settling down with Ash would obviously have a significant impact on Ash's story since it involves him directly. It's a logical progression in Serena's story, but completely random in Ash's at this point.