As Rick Daly says, Bob Shane was the voice of the Kingston Trio.
Ironically, because of their hit "Reverend Mr Black", John Stewart ALSO became the voice of the Kingston Trio in 1963.
This was apparent on their follow up single of "Desert Pete" and the album "Sunny Side!"
John Stewart was the lead vocal on 6/11 songs:
Ballad Of The Thresher
Blowin' In The Wind
Those Who Are Wise
Rider (shared with Bob Shane)
I agree with John, but it's more than the quality of Bob's voice, it's his versatility. The guy could jump from a murder ballad, to a Broadway show tune, to a sea shanty, to a ballad, and back to a raise-the-roof rousing song, and do all of them equally well. Name any other folk-era singer who could do that.
If Bob had left, Werber would never have chosen John as Bob's replacement. He would have gone for someone he thought fit the lead singer/sex symbol image.
The Nick, Dave and John group was an idea floated in the 1970s. They would have made some interesting music, and probably some innovative music, but it would never have been Top 40 stuff. It would have been fascinating to see Dave and John combine their creative energies. The one time they did that -- on "Ride Stone Blind" on one of John's albums -- the result was hauntingly beautiful.