The Swedish audiobook streaming service Storytel does allow synthetic voices in its library. On this page, you can listen to an example in English. I think it sounds pretty decent with a dynamic voice – complete with breathing noises! Imagine what it will sound like in the future.
I think there will always be a market for authentic voiceovers. I, for one, will happily pay a premium to get a non-synthetic voice for my audiobooks today. But tomorrow? I don't know.