You're absolutely right about Slash. (In fairness, almost no one can sound quite like Tony. There's a local tribute band in my neck of the woods called Sabotage whose guitarist really has it IMO, but that is an exception to the rules. Even among guitarists who can technically run circles around Tony.) And Axl is not exactly cut out for mid-70's Sabbath, at least not anymore. I never really loved his voice, even in his heyday. But I still think they carried it off in rock & roll style. It felt like a genuine tribute to me.
Anyway, I don't think most folks in the audience would want to hear one star player/ vocalist sound like another one. Excepting the crazy virtuosic chameleon hired gun types like Nuno, they want to hear Slash sound like Slash, see Axl be Axl, etc.
(That said, in about 2 hours, my little pick up band is going to have a couple of beers and play a better version of SBS to no one in particular in my basement.

)