I think that's a fair definition, although I'd be even more strict in my definition. I saw Bob, George, and Roger Gambill, and thought they were awful. There was a lot of vocal commentary post-performances, from a wide community. A number of reasons for that, which I won't amplify in this post. Jump ahead to the late 1990's and the group is Bob, Nick, and George, and most definitely "The Kingston Trio". Clean, tight performances, aged but thoroughly the group people came to hear. Sounded right, behaved right. You'd take your children and Grandma to hear this K3.
I think one has to include professionalism, limiting the material to the expected character of the original group. The groups are older, and so are we, so you expect changes. At some point the original group piddles out. And "original sound-alike group", isn't the real deal. That's not to demolish tribute groups as irrelevant. It simply says, what's real is real and what ain't just ain't. There can be a tribute group with one original member still involved, say "The Fifth Dimension", but without the other members, it isn't original.