I think the only issue here is if those three without Robert Plant did tour as Led Zeppelin, would people show up? Would they fill the seats? Of course the guys would be SLAMMED in the media and by many fans, but so what? If the guys are having fun every night and making money and some fans turn up and enjoy the show, with someone else singing, who cares? The Eagles perform TITTL, to much resentment from Randy fans, but who cares? LZ would be an entire show of that!
I for one really enjoyed hearing Taylor Hawkins, the
DRUMMER from Foo Fighters, sing Rock 'n Roll in Wembley Stadium with JPJ and JP and Dave Grohl absolutely wailin' away on drums - it was awesome! He doesn't sing like Robert Plant and wasn't trying to - they were just having a blast up there. I wasn't offended that they were playing that song without RP.
https://www.youtube.com/watch?v=WAFciwFJrg8
I can't remember what channel it was on (VH1), but I saw a tribute to Jim Morrison that featured some of the guys from the Doors and different lead singers filling in. I remember Ian Astbury from the Cult, a fan who sounded just like Morrison (a la Journey), Scott Weiland and others, and it was great. Obviously, he is dead and Plant is NOT, so that makes things different, but I really enjoyed the show.
I have no problem with Robert Plant not wanting to do it, but the other guys shouldn't have to sit home. I guess that's my take. Get some rad singers for a tour, male and female, and let them have at it. The fans who think its a travesty will stay home, and those who want to hear some LZ will go.