Marc Guggenheim’s explanation at a recent Q&A event (via Green Arrow TV) may help Thea fans rest easier.
If she’s only contracted for 14 out of 23 episodes, she has flexibility for other areas of her life.
Arrow isn’t necessarily in the process of writing Thea out or replacing her altogether with a crew of newbies; Willa Holland’s availability has simply influenced how and when they can use her character on the show.