He can't go back to that year in that place or risk nuking everything. But, he could easily go back to a year later than that date, or a time far enough ahead that would be safe to go to (because lets face it, New York isn't off limits forever, Moffat wouldn't create a location limitation like that when the show is increasingly popular over there).
Also, Amy and Rory could have gotten on a train to New Jersey or something and he could have picked them up there, or he could travel there a year before the incident, wait it out, then pick them up.
Too many solutions for me to care about their demise. Actually, I lie, the cut scene with the letter to Rory's Dad made me tear up, but since that wasn't actually in the episode...