I imagine Sir David Attenborough would basically just cite the intro of "The Gods Must Be Crazy" movie, casually explaining how humans technological progress is futile in regards of happiness. That text seems to holds up to present day.

Actually, when the sad day comes and he passes away I'd be very comforted to hear his voice on new nature documentaries. I just can't watch them unless he's narrating.

I agree. This leads to an interesting question: can the estate of a deceased person sell or license their voice rights for new future performances? I suspect the law has some catching up to do.

This is not qualitatively different from existing situations, and will be only a minor legal wrinkle. Estates have been licensing the likeness of dead people for commercial purposes for a good while, now those likenesses are simply more sophisticated.

I'll tell you what will get complicated, copyright holders complaining that their product was used as input for the training algorithm and demanding a slice of any profits because they made the famous individual more famous by casting them.