I think in the U.S. it's also very important to think about health insurance and retirement.
An awful lot of the writers I know in three separate fields—genre fiction, consumer technical, and textbooks—have a spouse with a day job, or they have a day job, entirely because of insurance and retirement / pension benefits, direct and indirect.
This is especially true when you think of the potential to live to 90 or more. A spouse or a job with health benefits after retirement is a huge asset.
Health insurance for writers and other self-employed people is available; it's prohibitively expensive, and typically, while it doesn't suck, isn't fabulous. High premiums and deductibles and less than stellar coverage is typical.
If you've got pre-existing condition, say diabetes, a heart problem, previous cancer, lupus, etc. it's dismal even finding coverage. It's possible, but you're not writing while you look.
You can save for retirement via SEP IRAs and other means. Even if you have as spouse with benefits, writers should max out a retirement if at all possible, as well as a spousal IRA, and take advantage of things like medical savings plans.
If you have dependents, it gets even hairier.
So an awful lot of writers keep a day job or a spouse does, just for the benefits.