But Plant was kind of a hack live in the 70’s.
Um, agree to disagree there. 
I'd say 'hit or miss' better sums it up—I've heard dozens of boots from the later '70s and I'd say he was 'on' more than he was 'off,' and Page had just as many cringe-worthy shows.
I'm usually in lock step with Brother TAC, but on this one, not really. Plant had his off nights, there's no question, but when he was on, WOW. I went through a deep Zeppelin phase a while back and came out with a really profound admiration for him as a singer and a frontman.
One thing I will say is that to compare Robert Plant on the '75 tour of Zeppelin with the Plant of, say, the Carry Fire tour. It's a different animal entirely; different music, different band, different approach. I happen to like the current Plant BETTER than the histrionic rock god Plant, but that's my personal preference.
The problem with James - to the extent there is a problem - is that none of those caveats apply here. It's NOT different music, it's NOT a different band, it's NOT a different approach. Having said that, I rarely bail on a band because of the singer; I get that times change and voices change, etc. Bruce is not the singer he once was (though, once he's as good as he ever was, if that makes sense). Paul Stanley is a shadow of his former (vocal) self, but then again, he's still Paul Stanley. I got into DT in large part due to James, and he's still a catalyst for the music I like (I listened to 6DOIT the song yesterday and WOW), so I don't want to see him go. If we decided that change had to be made, and I had to pick one, I know who it would be, but I'm not at liberty to say right now.