Ash did seem to improve a lot because May and Dawn were brought in to be the "beginners" that didn't know anything. Back then Ash was the beginner that didn't know anything so Misty and Brock, as more experienced trainers, had to explain to him and the viewers the basics of Pokemon.
I feel like they only wrote Misty out because they needed to introduce Pokemon Contests. She was already a well established battler, not a coordinator and to suddenly introduce her to contests would not seem right. She has said before that she loves water Pokemon because they are beautiful and contest battles are about beauty and appearances.
With all the new elements introduced to Pokemon and everything that has happened Misty does seem like the least important female character. I hope we get to see some of her before Ash sets off to Isshu.