IG following doesn't translate to sales.
Sara might have one of the largest numbers of followers but that doesn't mean her followers are all shopping from VS and giving them profits. ex) Elsa has one of the slowest growing IG's out of the top girls (Sara/Jo/Taylor) but she must sell well otherwise VS wouldn't be pushing her that hard. Same goes for Jasmine. She has a third of the following of Sara but she ended up with the FB and is used considerably more. IG numbers really aren't telling of how well a model performs for a company.
I doubt Sara will go anywhere anytime soon, but I also don't see VS putting her at the forefront either. Also as mentioned, Sara has a solid career to fall back on. VS helped open the door to HF for her I think, and now she can grow her career on her own. Moving farther with VS wouldn't be a huge accomplishment in terms of growth even if it would be nice for her since I know VS was one of her big goals.