Point of information: back before I got the opportunity to write novels for a living, it took me 3-4 years to write one on a part-time/hobbyist/learning-the-art basis. That's because I had a day job, which came first.
As someone who is earning a living by writing, I typically produce a book a year, plus change (sometimes up to two books per year).
If downward pressure on production costs results in me having to seek work elsewhere, then of course I won't stop writing ... but I'll write a lot less.
Amazon's strategy of driving down prices by leaning on publishers to cap prices across the board works on the assumption that all books are created equal. It's commodification, equating work of serious artistic or literary merit, or work that lots of people like, with work that is neither artistic, meritorious, or even popular. If the monopsonist gets their way, then all authors take a haircut. Even if you're a market fundamentalist, this has problems: Amazon may be sending a price signal, but it doesn't apply to the individual SKU but instead to the manufacturers' entire product range across the board. Any useful information is thus lost in the noise ...