As I read through it, one comment she made, that a trip to the dentist was a desired luxury item, struck me as pretty terrible. Lots of folks think about a new car as being a desired luxury; but having a tooth filled? To me, that seems like it's a basic neccesity of life. I'm sure that you can agree with me if you've ever had a toothache.
I've started and operated several businesses; I've been self employed now for around 20 years; I have a pretty good handle on what it takes to make a business profitable; and even with that experience, farming has its particular challenges in that regard. But it's not much different than any other business, too.
Here's the bottom line: If you can't figure out how to make a profit on a spreadsheet, you will not ever make a profit. Even sometimes when you do figure out how to make a profit on a spreadsheet - you figure out something that looks like it will work, well, sometimes that doens't work out. But at least you have some hope.
If I weren't able to have reasonable confidence that I can have positive cash flow from my farm - I'm producing more money than I'm spending and have a choice about where it goes - that's the day when I stop producing food for other folks. It would be a lot easier to grow homestead-quantities of food; and I wouldn't need the tools and infrastructure that I use now to keep my 500 pigs and 50 cows and 200 chickens and all of the other critters that I have at this point. I could get by with a couple of cows (one for beef, one for milk) a few pigs (a boar and 3 sows) and 20 chickens - because I like eggs.
When I read this essay from this vegetable farmer I don't think that she ever did a spreadsheet, and I'm pretty sure that she's living a lifestyle that most of her customers wouldn't even consider. Is that really the kind of farm that you want to run? In her essay she's having fundamental doubts about it herself -- but that's something that I think she should have worked out years ago - before she turned over her first shovelful of dirt.
Sustainable to me means that the people working on the farm earn enough money that they can move themselves ahead, make their own progress. That's the goal I work towards in my farming.