I get it. If it's not there it's just not there. If Plant ain't feelin it there really isn't anyone to blame.
Exactly people will have to eventually realize that the 'one of concert' a few years ago was probably it for Led Zeppelin and a fitting way to bow out.
Robert knows his vocal limitations now would definitely affect performing the old songs live in front of an audience, unlike Jimmy who's guitar playing sounds the same as it did back in their prime.