Hmmmm. I really think it depends upon whether the band is any good with the replacements.
For example, I saw the Yardbirds without Jeff Beck, Keith Relf and Paul Samwell-Smith. The only two original Yardbirds they had were Chris Dreja and Jim McCarty. But they were great! They nailed all their hits and sounded fantastic!
I've seen Eric Burdon & the Animals four times in the past six or seven years. The first and last times they were great despite the fact that Eric's voice continues to lose the power it once had. The other two times they were disappointing. It all hinged around whether his instrumental backing did justice to the original songs.
The Scorpions, Deep Purple and Genesis sounded just fine to me despite the lack of some members.
So long as the fellows in Zep can replicate the thundering instrumental work that I liked about the band, all I think they need to reform is a singer that can hit the highs with power. I'm not in any way wedded to Robert Plant out front.