Has anyone ever tried listening to a computer reading something?? Its not the most pleasurable experience. It hardly compares to an audiobook. Honestly, I really don't think they would see any loss in sales of audiobooks due to people using text-to-speech software. You can buy eBooks and you can use software to read them to you on your computer, you've been able to do this since the advent of eBooks. So, have they seen a loss in sales of audiobooks with the advent of eBooks?? I doubt it!
The people that will actually use this feature are the people that need it. Very very few fully sighted people would use this. A visually impaired person, however, currently has to either wait for a book to be reprinted in braille or wait for it to be made into an audiobook - which can cost ten times the cost of the original book. Plus, only a very small number of books even get made into unabridged audiobooks. Visually impaired people don't care about how natural the computer voice sounds since its all they have - they're happy using such computer voices to use computers in general already.