Stephen, I think it is a very unselfish act to offer to bow out of the quest. But what one needs to not forget is that people join a quest and get involved at a certain level, perhaps even so involved that they become a critical aspect of success on the quest. Furthermore information is usually learned in a progressive manner and people learn how to work together and get to know each other.
Now what I am trying to say is that even though I appreciate people being unshelfish to bow out of a quest, this might well lead to information being lost or even a quest being failed. We all know everyone wants to be involved in plot quests and everyone cannot necesary be, but I think rotating positions on a plot quest could very well be detrimental to the overall outcome of the quest.
With that in mind everyone need to make up their own mind if they are needed on a quest and will make a positive contribution. The plot quest is not about getting XP (and that is not a stab at anyone) but rather of doing something that will impact the world in a large way. And unfortunately not everyone can be part of the important event. But everyone can have an impact in other ways (and here I mean possible side quests that will be run which will impact the main quest).
Now, I am sure that, for the most part, those who were prominent in the last campaign will be happy to sit back and support the efforts in this one without necessarily being the driving force. I can guarantee you that there will be lore, events and NPCs in this campaign that have their roots in the previous. And guess who has the background on those things?