yup, there are certain irreplaceable people in bands. They were both main songwriters and both contributed hugely to the sound.

Bon Jovi never would have worked without Jon or Richie, but I think others in the band could have been replaced if necessary.

Iron Maiden changed singers and sounded the same musically, but Blaze was nowhere near the vocalist or songwriter Dickinson was.

There's a ton of more examples where it did and didn't work though.

Going back to Giant there is not one song on that album that sounds like anything from the 3 Giant albums, everything that Giant was left with Dann.