Nothing would surprise me at this point in their career. If there had been a band meeting about retirement, even with the excitement of the reunion, I think we would have seen or sensed something in their tone or mood. I get the sense that it's not one and done.
A scenario that seems more plausible is that 1 member - could be any of them - said, 'I've only got one tour left in me' and this prompted the reunion. Doubtful but who knows. And in this scenario, if the others felt like continuing, the retiring member would have to be replaced.