I think JP, JLB and JR are irreplaceable. JP, for being the main songwriter as mentioned, and thus a key part of their sound. JR because nobody else can do everything he does without seriously compromising the keys, and because he's been such a huge asset to the band's sound, and JLB because he's so identifiable as the voice of the band.
JM and MM are awesome, but I don't think they're quite as irreplaceable a component of the band's sound, especially with MM being new.
But ideally, I don't want any of them replaced!