Don't forget Ozzy..... When I was a kid and listened to "Live at Last", I was like "who is singing this stuff?". And this isn't just a live show, it was an album release. I've heard plenty of live vocals that wasn't even close to the recording.
I think people say "fire them" easily because they have nothing at stake. They don't need to break the news, they don't need to deal with the relationships, they don't need to find a replacement or deal with ownership issues like songwriting credits, etc. If each TB member had to actually deal with the fallout of firing someone, I'm sure it wouldn't be so easy to say.
I've done music for over 20 years, and I can count the number of people that I've fired on one hand. I know a bandleader around here who will fire and replace someone as soon as there's any type of issue. He is the epitome of the TB "fire them" crowd. He's had this band for years, yet bands with less laps around the block do better. He's run through most of the decent players in town, and lots of people won't work with him. He has clients pissed at him because the band he brings is not even close to the band he advertises. He has gigs booked for months in advance and they are paid, but he still posts ads on Craigslist to find someone to fill a gig last minute. By following the "fire them" philosophy, he really has stunted the growth of his band and diluted the quality of his show.
Also, we don't know what else he brings to the band. If he wrote the songs, then by firing him, you'd have to write all new material unless you are going to steal his tunes (bad karma). If he has the PA, booking connections, etc. you may send the band back a few steps. IME, you should find a way to work with a person before letting them go.
IMO, the biggest problem is that the phrasing that he uses highlights his weaknesses too much. It probably can be fixed. If I can sing lead, and actually have a crowd appreciate it now and then, anyone can.