The sad fact is is that writing as a business is a lot like construction work. There are good times and bad, and just when you think you're at the top and things are smoothing out, things can take a wild dip. If you're not prepared for these, you're going to be caught by surprise. There's really not a lot of stability in careers in the arts.
Reminds me of that article about the woman who spent as if the money was going to keep coming after she sold some books. If you've ever read any of Lawrence Block's writing books, you'll probably be prepared. He's very open about the ups and downs he went through in his career.
Anyone who can write a book a year and make good money should be proud of that--and a little cautious. It could continue, even get more stable, or it could not. I'd be planning for it to not. :D I'd try to get some books in the bank, so to speak for a dip.
One book a year for most people is really not going to cut it long-term. That's the top 1% of authors or an even smaller slice of the pie of all authors earning a living exclusively through writing. If you aim for that, you're probably going to be disappointed. Maybe not, but... yeah. It's like deciding you're going to make a living on lottery winnings. :D
Four books a years is much more common in the earning a living group, and more than that for a lot of people if you count in pen names and the like. And some of those people are not depending 100% on book sales for money either.
Truly, it's different for everyone. If a few books a year don't work, try more, and if that's not doable, try something else until you find something that does work. And if one a year is all you got in you or all you're willing to do, then try anyway and hope for the best. That 1% is made up of people and you never know. Could be you. Just try not to lose hope if it isn't.
I love writing for a living, and I've had some years at this point where I put out one book and made a good living and some where I've put out four and had to scrape by. It really depends on the book, the series, and the pen name and the amount of money I happen to need to get by. :D
Up or down, I wouldn't want to do anything else. If writing a book a month is what it took, I'd be trying a lot harder to write a book a month. As it is, I'd love to be there anyway, because I'd love to finish everything I've started and write a whole lot more besides, long before I die. :D
Lecture done!
TL;DR I don't pander to non-readers. ;D 
This is why I think financial independence discussions should be a vital and ongoing conversation amongst writers actually making a living at this.
The flood of gold needs to be preserved, thrown into index funds with an aim of having enough to live off if Amazon boots you.
I'm acutely aware I'm entirely reliant on Amazon. I've made money wide but the other stores are a joke compared to Amazon. Even diversification to audiobooks is still just Amazon and the recent ACX ban just shows how quick it can be taken away.
Years ago I had novels that reliably would make $50K in a reasonable time. Then in came KU and now the same novel makes around $20-35K. I can sit and work out the pages read and convert them to sales and my novels now still are on level with the pre-KU ones. The only difference now is that instead of making 70% of $4.99 per sale, I'm getting $0.0045 per page and the rest of that money stays in Amazon's pocket.
I'm pretty cynical about the next five to ten years. I don't think Amazon will ever stop reducing royalties. They started with 35% and only went to 70% because Apple entered the game.
But they can read the landscape. They cut audible from 70% to 40% and there's not much anyone can do. They are the biggest game in town and other places have even worse terms for weaker distribution and sales.
I'm coming out of a period of disrupted writing due to having two children and now I'm starting to pedal like a motherf*cker again. Plans of stock up on manuscripts, release one a month have been replaced with get them out as fast as possible, take the royalties and keep them in the bank.
I really don't trust Amazon at all. If they could Spotify us with royalties they would and may still. It'll just be some new per page calculation model that will come out and suddenly that $0.0045 per page will be a fever dream as payouts will be 1/3 or even less than that.
