It's already possible to tag (parts) of sentences on how tts has to speak them, quiet, excited, etc. Soon it'll be just a matter of what is cheaper, a recording or a tagger/director, and that tagging will be partially automatic. As with many things absolute quality, sadly, won't be the target, good enough to keep people buying the product is.
Not sure how I feel about this prospect, I love audiobooks, and for big titles the narration cost is a drop in the bucket, and I like the idea of real narration. But at the same time lots of indie books don't get an audio version, and that tts allows more books the have an audio version is a plus and having each character get their own voice works be nice.