Again, Lynx, the job of a rock band is to perform the same songs every night (or every tour, or whatever). The job of a writer is to do something different.
And even bands... there are tons of examples of where your analogy falls flat. The Dave Matthews Band saxophone player recently died. They replaced him with a sax player with a VERY different aesthetic. The band sounds quite different now. But, it's not the end of the universe; it's not better or worse, just different, and it's fun to hear new spins on the old songs. Change CAN be good. You're acting like it's an accepted fact that any alteration in the members of a band is clearly ruinous, and that's just a complete exaggeration.
Look at it from a business standpoint. If you just keep doing the same thing over and over, the same predictable stories with the same characters, there's only one place your sales can go (speaking in the long term) - down. In order to attract new fans, or give old fans who got bored with the old paradigm a reason to come back, you have to do something a bit different.
There's not much money in giving an ever-diminishing group of people bland comfort food stories.