I think this should be viewed from different perspectives...
If you look their chemistry, as a group people co-existing with each other (in studio, on tour), none of them could be replaceble. For example, it looks like Nicko being the drummer would be the easiest to replace, but overall looking it would represent deep void in a way that the band funcions together (considering his character and his impact on the band).
From a studio perspective, any of them could be replaceble, as they were in the past. Altgought, in last 15 years Bruce, Steve and Janick are irreplaceble IMO. Considering Book of Souls, Adrian and Nicko really kicked up a notch, so I guess they stay in, as well. Which leaves Dave, as I've noticed for some time now. I just don't see his contributions on a record in a long time.
Gig-wise, Bruce and Steve are irreplacible. Since I love note-to-note playing (as it were in studio) Adrian also stays. And Dave. Which leaves Nicko (who I can not see the whole gig) and Janick (his theatrics were awesome to me, some 10 years ago, now I just wish for him to focus more on playing, less on dancing).
Overall in theory, it looks like only Bruce and Steve are irreplaceble. Of course in real life, if they replace even Michael Kenney, I would start preaching like it's the end of time.