My understanding of "The Five Doctors" is that, as a result of being picked up by the time scoop, each of the Doctors was temporarily robbed of all specific memories they had from each previous incarnation. So, taking that into account, once you do the math, the 1st Doctor is actually the oldest since he's the only one that can actually remember the 1st 400 or so years of his life.
Still, it might be funny if the 11th Doctor tries to take charge in the anniversary special by noting that he's the oldest, only for everyone else to burst into hysterics at the notion.
IMO, if Eccleston was really sick of being asked to do a reunion special, I think the quickest, most painless remedy would be for him to just do one! Then we'll all get it out of our systems! (Honestly, I think there would be far less fevered anticipation if they had already given us a proper multi-Doctor episode already, even if it were only 2 Doctors. But then, I suppose this fevered anticipation is exactly what they would want.)
I would also imagine that the large volume of work and relatively low pay (BBC TV role versus film role) would be a large turn off for an actor more used to motion picture industry!
At least he made it through an entire season, unlike Geneviève Bujold in ST: Voyager!
IIRC, Bujold's departure had as much to do with Berman & Paramount as it did with Bujold herself. I've watched the deleted scenes with her on the Season 1 DVD and they are pretty awful. She just couldn't quite muster the commanding presence necessary to be a
Star Trek captain, particularly the 1st major female captain.