I've listened to a lot of Books on Tape from the 80s and 90s where narrating in a mostly flat monotone was the norm, but even they would inject emotion and vary

I've listened to a lot of Books on Tape from the 80s and 90s where narrating in a mostly flat monotone was the norm, but even they would inject emotion and vary their delivery where appropriate. Good modern audiobooks are a true performance even when the narrator isn't doing a bunch of highly distinct voices. You can't replace that with AI any more than you can replace a great voice actor.