Thought it might be an interesting idea for a thread.
Lineup changes, Mark I, Mark II, this lineup was better, that one was worse, the reunions.... we've all been there with our favorite bands. Musicians and singers get replaced all the time. Is there however a musician or singer that you feel is, de facto, irreplaceable?
I don't mean it literally. ANYONE can be replaced, that's a given. There's no shortage of musicians and you'll always find someone who can play like someone else or sing like someone else, and in the case of singing, do it even better if they're young. But, is there someone who you think that, for whatever reason, is irreplaceable in the sense that nothing and no one else would feel "right" after him / her, making said band member "irreplaceable" even if there's always someone out there who could play or sing?
I think potentially more interesting answers might avoid the most obvious choices. Steve Harris can't be replaced in Iron Maiden and John Petrucci can't be replaced in DT, well, d'uh. Maybe our attention should be on musicians that are not the absolute leaders and main songwriters of their own band, but that still give you the idea that there's no way a band could be considered the same without them, to the point that their absence would be a complete and definitive paradigm shift impossible to recover from.