The Weeping Angels are forever on my sh*t list, I'll help the Doctor wipe out those time abominations!!
Still this is probably the best way to let them go, the last episode pretty much cemented the Ponds and the Doctor could not be seperated for long and as long as they were in the others lives they'd be bouncing about the TARDIS across time and space, so them deciding to NOT travel was rubbed out as a possibility despite some teases that was what the Power of Three would cement. So that left the next options of "they get stuck beyond the Doctor's reach like Rose did", or "they die in their travels" and the latter would I think have destroyed the Doctor beyond recall. Even if they did fully thwart the Angels we knew the Doctor was never keen on watching his friends grow old and then just up and die one day leaving him alone, so in a sense what happened spared him that agonizing Hell, but either way it was gonna suck.
Still my hat's off to how they did it. They're gone but they had each other and a full life, and made sure the Doctor understood there was no regrets, they loved him as they knew he loved them, and most importantly Amy left these words (which River echo'd)
"Find someone, don't you dare travel alone." Very sound advice.
Farewell Ponds, live, love and you'll never be forgotten.
Cuz that daft old man in his blue box never forgets...
Aye which is what does ultimately make their exit stage left more tolerable. They were together, and the Doctor got to have decent closure that didn't leave him a mess like after what happened with Rose (stuck in a parallel world where he can never reach her, and never got to tell her he loved her), and Donna (forced to erase their memories together least the Timelord bits she absorbed destroys her mind, costing him his best friend). The Waters of Mars showed us truely how far the Doctor was ready to fall when he lost so much and didn't have someone to regin him in. Thankfully despite the sadness the Ponds and the Doctor got some of the best "goodbye" closure of the show, and of course the tears flow more when the end scene rolls and you realize at the end "Oh sweet lord he DID go back to see little Amelia" when they roll that footage with Amy's voice over telling him he has a little girl to tell stories of the future to look forward too and you go back to young Amelia hearing the TARDIS noise and smiling, it wasn't a spur of the moment scene from past seasons he did go see her again and fulfill his promise, a bit late but he kept it to that sweet little girl who'd grow up to be so much more.