Moffat is pretty obsessive, I've found, about tying up loose ends, so there's a good chance we'll learn more about why the Ponds stayed in the past (instead of requesting pickup elsewhere/elsewhen), and the Doctor let them. Experiencing the 20th century up to the point when they were roughly born, the part they mostly knew from history, and having seen the future, who's to say they didn't wanna stay and live a time adventure sans Doctor? They were gonna leave him anyway. And he was beginning to realize they would end, making him likely to just stop visiting at some point. Did I hear that line correctly, had they really spent ten years traveling with him, with roughly 2 years passing in their present-day?
Anyway, WWII America would be an amazing time for a present-day nurse to show off and teach his skills, and a head-strong feminist to join/lead the women-in-the-workforce movement. Maybe they were so important they did become fixed points in time. The 60s? Weed was mild and chill, and occasionally dropping acid meant they would still get to trip their balls off much like in a time vortex.
Alternatively, the angel fed on both their time potentials, possibly meaning they could no longer travel in time. Amy got fed on willingly. Doc got the message. Though, did people sent to the past in Blink get time locked when eaten by an angel? Either way. I was puzzled by the angels suddenly learning to convert other statues. This outcome was certainly hinted at with the hokey ending to Blink. Hopefully we'll learn more about the how of it.