Keith Moon - we have proof that he was irreplaceable, since they never sounded the same (or as good) after he passed.
John Bonham - the band knew he wasn’t replaceable. Aside from a few shows, the band never did anything new after his death.
Neil Peart - similar to The Who, look at what Rush became after he joined. He had a huge influence on the band and elevated them far above the fun-but-derivative rock band of the first album into an iconoclastic act.
Lars Ulrich - here through sheer force of personality and vision. Say what you want about his playing, his leadership made Metallica into one of the biggest bands of all time.
I’m hard pressed to think of someone with more presence in his band’s sound than Stewart Copeland. Divide up the components of the Police’s musical personality and his gene contribution is a massive share. Everything in their sound feels so laid bare already, Stewart’s drums feature all the more heavily within their genetic makeup, from his tuning to his deliberately unconventional patterns and grooves.