I've been sacked that way - not because I was a bad bassist or my replacement was a better one (personally, I think he was worse), but because my replacement was a mate of the lead guitarist and drummer and he wanted to join the band. I wasn't at all happy.
Still, all was eventually forgiven and I depped for them recently when their bassist who was a replacement for my replacement was unavailable for a gig. But I wouldn't like anything like that to happen again.