Gutiarists can replace strings, update hardware, whatever, and as long as their dexterity is there, they can still play. Even if they lose something, they can compensate some other way.
Vocalists, on the other hand, don't have the luxury of replacing vocal chords. They can have them fixed, patched, whatever, but they're never what they once were.
However, they, too can compensate by fitting their style to their abilities.
In the field of 'big vocal' rock, (Hey, I think I just invented a new genre!!), I doubt singers like Plant, Roth, Uncle Ted, Dickinson, Halford, and all the other bellowers are thinking about what their voice is gonna sound like in another 15-20 years.
They're singing for right now, so they give it all. Oh, I'm sure they probably take some precautions, limbering up with scales, or drinking certain things, but nothing along the lines of a classically trained opera singer.
Anyone who owns the 'Live After Death' dvd can not only hear, but can actually see Dickinson struggle vocally through Aces High and Two Minutes To Midnight.
His chords weren't loose, and it took a couple of songs to warm them up and get them stretched.
Like an athlete with tight hamstrings, they need to warm up.
All that being said, if a Zep tour ever does come to fruition, these cats are certainly not the same people they were 30, or even 20 years ago.
But, these guys are professional musicians, and they'll make music that fits who they are now, not who they were in 1969.
Personally, I would looooovvveeee to see a semi-acoustic (don't know when it was renamed unplugged) show from Zep, like Neil Young's Rust Never Sleeps.
Can you imagine, Achille's Last Stand, or The Lemon Song, or Misty Mountain Hop just a couple of guitars, maybe a tamborine?
Groovy.
I'd go right to the bank and take out a loan and get tickets for that.