Also, I assume this is the last we see of Alex Kingston, since she lives backwards in time compared to the Doctor, so her introduction to him is his farewell.
But she doesn't live "backwards" in time compared to the Doctor - their timelines don't intersect in chronological order, neither backwards nor forwards. It's quite feasible that the Doctor's future timeline could intersect with hers again at any number of points between her regeneration into River Song and her death, either before or after previous intersections. The Doctor and River's timelines are tangled up like spaghetti.
We've already seen that they keep notes to prevent spoilers. If their timelines only intersected in inverse chronological order, that wouldn't be necessary. Plus they have the whole relationship thing to develop yet.