1. In fact it has been applied to the story of Ramayana too.
2. In Das'aroopaka too this comes as part of discussion of Vastu, Vastu in general and not Vastu of Rupaka alone. Vastubhedas such as Prakhyaata, Utpaadya and Mis'ra also , though discussed in Das'aroopaka are applied to the Vastus or Itivrittas of S'ravyakaavyas als.
3. S'aakuntalam is not khyaatavrittam. It is mis'ravrittam.
4. Whether the Panchasandhi structure helps in understanding Vastus of Roopakas alone or it applies to Vastus of other narrative literary works also, can be checked by literary scholars. There is nothing inside the theory which is roopaka-specific.
5. Comparable to this in the western poetics is the five part structure of exposition, rising action, climax/crisis, falling action, denoument/resolution also in its origin comes from the theory of Drama. Is it not applied/applicable to other narratives?