With how long it took for Sinon to get to that point, it probably wouldn't make sense for her to be the character to take Liz's spot. Liz also has SAO history with Kirito and Asuna, so she's easier to bring into the plot and start influencing people. Sinon in comparison was kind of like a cameo to me back in Part 1, considering her canon story arc never happened. But anyways for the sake of limiting the characters and having a finer focus, it would have been fine for Sinon to just be a town bike off on the side barely getting any scenes.
Side note, I'm very happy with her current scenes, but definitely recognize she's one of those characters that wasn't needed alongside Alice.