I'm not talking about upgrade. Look at the subject of the thread.
There has to be a line drawn somewhere. If someone fails, retrains and then fails again, maybe they should be let go. Obviously this is a case by case basis but there has to be a breaking point.
Face it, people lose their skills. This isn't a hamburger we're talking about. Pilot's can't let problems on the outside affect their performance. It's impossible to forget ALL of life's stresses once the door shuts, but one must learn to manage stress.
Would you want your favorite football team to keep playing a former MVP just because of his past *cough* Warner *cough*? Eventually, coaches must swallow their pride and forget about the money they spent on underperforming players (aka pilots) and just bench them.
Upgrade is a different story, then yes, I think you should go back to your FO seat. But, if you go back for upgrade again and still can't make it, time to throw in the towel.