If you go by the "Christianized" version of the story, the grail is returned to heaven, because that's where it belongs (it's god's grail, and doesn't belong to humans).
...for in that place was part of the blood of our Lord Jesus Christ, that Joseph of Arimathea brought into this land, and there himself lay in that rich bed. And that was the same spear that Longius smote our Lord to the heart; and King Pellam was nigh of Joseph's kin, and that was the most worshipful man that lived in those days, and great pity it was of his hurt, for through that stroke, turned to great dole, tray and tene.
...
And therewith he [Galahad] kneeled down to-fore the table and made his prayers, and then suddenly his soul departed to Jesu Christ, and a great multitude of angels bare his soul up to heaven, that the two fellows might well behold it. Also the two fellows saw come from heaven an hand, but they saw not the body. And then it came right to the Vessel, and took it and the spear, and so bare it up to heaven. Sithen was there never man so hardy to say that he had seen the Sangreal.