if Priest kept their classic lineup in tact we probably would be having similar discussions and Priest would have retired by now (probably around 2012 and definitely by the time Tipton was diagnosed with Parkinson’s). I think they don’t get discussed the same way because they are willing to replace core members in order to keep going. Iron Maiden so far hasn’t, but if they announced the next tour would be a standard greatest hits tour and that Nicko would be stepping down, the expectations would probably change to Iron Maiden sticking around a few more years. As it is now, it seems like they are going to retire with this lineup and Nicko’s age/health makes it hard to imagine them going beyond another 2-3 year tour cycle.I'm always not sure why so many people are certain that the upcoming tour is Maiden's farewell tour, whereas there's hardly any such discussion about Judas Priest, for example.
Rob Halford is also 72 and has a demanding role in the live performance. I don't know if I really find that logical.