As I pointed out later in the thread, both the Doctor and the Ponds were both
looking for a way out of the relationship they had, and they both knew that there was little they could do to make that happen.
This solved that problem for all three of them. The moment the Doctor realized that the Ponds lived a long and happy life together, that was that. The dangers inherent to trying to "rescue" them far and away exceeded the actual desire
to do so.