That being said, it would probably be in Yuna's best interest to not attempt 2018. In five years, several of the younger girls could easily be at or above her level (Tukt, Sotnikova, Gold, Osmond, Li).
I can't read the future, but Yu-Na has a lot of pride in the technical side of her skating such as her triple/triple combination and her lutz, etc. I don't think she would want to come back to competition unless she still had her big tricks--which aren't easy to maintain, even though Yu-Na makes her 3/3s look easy. A lot of extremely hard work and training goes into those 3/3s and I doubt she'd want to maintain that level throughout her 20s. She wants to go to Sochi and finish her career on a happy note. I don't think she wants to drag out her career forever--she'll go for show skating instead.