Maybe front man isn't the right term for the ones you listed, more like showman.
I read an interview with Plant in RS many years ago about front men and he made an interesting comment. He said people come to hear our music performed differently since it is live; they do not come to see it. We perform our music with the best musicians in the world, that's what they come to see. I took that as why he really wasn't a showman and took his voice to be one of those instruments and I agree with that.
Guys like Mercury, Morrison, Cooper, Roth and Ozzie are more showmen than front men. I also think one of the developing problems with Van Halen was Roth's overshadowing the real star of the band and pulling attention away from him. I don't think Plant ever saw himself as the star, he reserved that for his friend, Page ,and he was the first to call Bonham the best drummer in rock history. The last special he did on ACL, he would draw attention to his players. His respect for those that could really play was apparent.
The showmen you mention are in the minority, many more front men and women do not draw particular attention to themselves while performing live and see it more as a band presentation. But I like those showmen too, Alice Cooper was the best and most creative I'd ever seen and since he really didn't have a great voice, he needed that edge.
It's really semantics to me as I am not using the term front man as you are but more as to the contribution of that person to the overall effect of the music and in that area, Plant has no peer to me. His vocals were every bit as much of an instrument as Page's playing. I can listen to his voice soar on Immigrant Song over and over and never tire of it.