Manning should retire. His trajectory the last 2 years is strikingly similar to the end for the great mountain man Bert Favre. Old age strikes fast...and often with startling impunity.

Favre
2009 - career season...33 TDs, 7 INTs...only season over a 100 QB rating in his career. Looked like he could play for 3 more years.
2010 - looked every bit like a 40 year old man who had absorbed a ton of abuse during his career.

Manning
2013 - career season...55 TDs, 10 INTs...second highest QB rating in his career. Looked like he could play for 3 more years.
2014 - during the second half of the season looked every bit like a 38 year old man who had a surgically reconstructed shoulder.

If he comes back, it will not end well. He'll look OK here and there, but his arm is gone. He gets by entirely on knowledge at this point, and even that wasn't enough to get by the last 2 months.