There's a weird separation here--day job versus writing full time. If you're living off your writing, that *is* a day job. Complete with tracking income, paying taxes, managing budget, and worrying every time you get a pay cut or the income goes flat (guaranteed to happen). That's in addition to learning marketing etc. Even if you get picked up by a publisher, now you get to learn contract law. In no scenario do you "just write."
So the question, when considering quitting the job where the pay is assured, payroll deduction just gets handled, and someone is contributing to your IRA or whatever, ... wait, where was I? Oh yeah. The question is this: do you want the whole job? Not just the writing part, but all of it?
And another thing, just to cheer folks up. We think of the big successes and in our worldly wise minds we figure that ain't likely to be us, however pleasant the dream. Fine. So what is the likely reality? Scraping by. Or, still more disheartening, making it for a while. You quit your job because you're getting good income from your writing. You go along for a few or even several years. Then, there's a slump. Maybe a recovery. Another slump. Or just a slow, steady decline. Even flatlining ain't great because expenses keep going up, don't they? And now you're faced with having to try to get back into the work force (as if writing ain't work!).
As has been said here, writing as a _career_ is the way to look at it, if you're thinking of quitting the day job. Will writing get you all the way into your retirement years? At the very least, it's a decision where you really ought to get solid advice from professionals (financial, legal, and the writing pros).
OTOH, running full-tilt over the cliff is always an option. It's been done.
