While I agree that at some time, characters get too high to play with anymore, I strongly disagree with your planned method of retiring. To me, it feels like a bad ending. My character would be gone, you left me no choice but to pick one evil. That's not very satisfying as a player. In real life, bad stuff happens, but at the gaming table, I want to be able to get up with a good feeling. 

To actually make it a **happy ending**, try to ask each player, what his character's happy ending would be. Some may want to rule a kingdom, or become gods, maybe the mage wants a library and a stack of potions of longevity, maybe the fighter wants to live a life in peace on a farm somewhere. Maybe, someone wants to die a heroic death. If you ask them, they will tell you. And you can craft some final adventures around those ideas. Take that evil kingdom from it's oppressive ruler and plant flowers everywhere. Sneak into the god of trickery's domain and install the thief as a new god. Stand against a horde of daemons, defending humanity and dying a heroic death while the rest of the party slays the master. Find a farm, a wife and happiness. Whatever. Once the final adventures are over, each character should be **happy to stop adventuring** because he has achieved whatever he wanted to achieve all his life and the players can create their new characters with the fuzzy warm feeling that they actually managed to "win" a role playing game.