When the band member who's passed is someone like Lowell George or Vivian Stanshall, you're going to have major problems replacing them - and not only because of their creative personalities.
In Lowell George, Little Feat had an outstanding vocalist with a clearly identifiable sound and a bloody good guitarist. Finding someone who'd match up in both categories would (you'd expect) be close to impossible and so it must've made sense to get someone in for the guitar role and find a dedicated singer who's reasonably close to the vocal timbre you're going to need.
So longtime Feat friend Fred Tackett looked after the guitar side of things and Craig Fuller took on the vocal bit.
When the Bonzo Dog Band got back together for a reunion back in 2006 they had all surviving original members there including Bob Kerr and Vernon Dudley Bowhay-Nowell, but there was no single person who was ever going to be able to replace the late, great Vivian Stanshall.
The solution, repeated when they went into the studio to record Pour L'Amour Des Chiens, was to use Stephen Fry, Adrian Edmondson and Phill Jupitus to tackle various Viv-substitute activities. Given the fact that it's Stephen Fry up there replicating something as a long term fan might tend to make up a bit of the disappointment that you're not seeing the man himself.
Then again, over the intervening years personal relations may have deteriorated to the point where two or more members of the outfit can't stand to be in the same building, let alone on the same stage.