One of the potential problems with long term campaigns can occur as a result of the characters themselves. Campaigns can be described in many ways, focused or unfocused, plot driven or sandbox, but all revolve around its unique cast of characters.
Over the last 15+ months I have been following an online campaign that is using the "Advanced" 2e system. I won't mention the name or "channel" because I am not going to be particularly kind in my comments. Frankly this campaign see4ms to have outlasted its vitality and this is evidenced by the players who no longer "show-up" for scheduled sessions and by characters who now seem to lack purpose and direction. In other words, it has become tedious and boring to watch and probably to play. This is not how this campaign began, however. In its early sessions it was interesting and engaging. The setting was revealed to show depth and surprise and the characters acted as explorers and investigators as well as combatants. Somewhere along the way, the campaign lost its way and devolved into playing characters who although they are getting more powerful, they also are very much less fun to play and watch.
Every character in every campaign has a beginning when we as players "roll them up". Some characters die a violent death during play. Some succumb to disease or level drain. Some get rich and retire, maybe to become a recurring NPC in future campaigns or to run the local tavern. Too many characters are just abandoned, their ultimate fate lost to anonymity, largely due to the campaign being abruptly abandoned.
There comes, even to kings, a time of great weariness. Then the gold of the throne is brass, the silk of the palace becomes drab. The gems in the diadem and upon the fingers of women sparkle drearily like the ice of the white seas; the speech of men is as the empty rattle of a jester's bell and the feeling comes of things unreal; even the sun is copper in the sky and the breath of the green ocean is no longer fresh. – from “The Mirrors of Tuzun Thane” by R. E. Howard 1929
At some point we all desire to play something new and our old character loses its appeal. Sometimes we just feel that we have exhausted all the interesting possibilities this character presents. Our PC's "story" may have been told and we have reached a logical stopping point with them. Maybe we are just bored with our PC, or maybe the "latest and greatest supplement" has given us an idea for a new and improved PC and we are looking to start anew?
In many games the characters are there simply to explore the fictional world - the setting is the central element of this type of campaign and PCs may come and may go without serious consequence to the on-going campaign. This may seem "natural" and is often how the real world works for us adults as acquaintances come and go. Children may view the world, and everything in it, as being there merely to meet their needs, but we adults quickly learn that this is not the case.
In a role-playing game where the PCs are the focus of play, the dynamics of an evolving "story" can be quite different than in a sandbox style game where the setting is being slowly revealed through exploration - during which play may involve many quests, each of which adds to the compiled lore that enriches the setting.
What seems logical to me is that a PC focused campaign should be planned from the beginning to be of a relatively short term and should be goal oriented. A specific task for the PCs should be clearly defined and the cast of characters (and their players) devoted to accomplishing the task. This is quite similar to a novel such as the famous "walking the magic ring to mount doom" which has a beginning, middle and end. The character(s) should be facing some crisis which when resolved will complete the "story" and therefore the campaign. Wrap it up nicely - then move on to the next campaign.
Regardless of our reasons for retiring our old character, I would encourage us to see the character deserves a decent send-off. After all, has not our well-played PC given us hours of enjoyment, and probably provided us with some laughs along the way! By giving careful attention to exactly how the PC will "ride off into the sunset", we can nicely wrap up this particular chapter in our emergent story - and perhaps provide a plot hook for some future adventure.