Sayu has come very far. She's really earned her lines. First she was god awful. I hated her solo in Platinum 9.
Then she was Robo Sayu and really caught my attention in songs like Fantasy ga Hajimaru.
Then we got to the Sayu we have now which is sometimes Robo sometimes not but always sounds amazing. I love lalalala no pipipipi and her Aruiteru sounds amazing from the preview.
So she did ask for more lines and slowly she got them, but she also earned them. From the sound of it you have to improve in order to have your request granted and it pushes them to work harder.
Sayu was complaining about lines after Resonant Blue and she got like a short line or two by Naichau Kamo and I think a few lines by Nanchatte Renai if I remember correctly.