I've seen it happen with other people -- I considered doing it myself on more than one occasion; quitting the day job, that is, to dedicate my time to write.
None of those to whom I mention above who stopped working to write their books were successful.
I can only speak for myself on this subject... So, here goes.
I have long dabbled in the "Go into business for myself business" These endeavors included land- scaping, cards and crafts, t-shirt art and clock-face art and even tried my hand at carving whirley-gigs! We had some moderate success selling the t-shirts until the bottom fell out from under us during the last bad recession which caused business to dry up. The other go into business for myself attempts fell apart due to lack of funding, lack of experience and lack of clientele or getting into a field that was so saturated with competition that it all but insured our failure.
Through all of this experimentation, I held on to my jobs that I was working at the times. I had to have some kind of income coming in while I was trying to become Rob the entrepreneur.
More than once did I start to think that my "real world job" was getting in the way of my aspiring enterprises as alluded to above, I considered quitting.
Wise that I didn't
I was paying a mortgage, I had lot rent to pay for and don't forget insurance and the monthly bills. Had I have quit, where would the money for these things come from?
After I published my first book, The Warriors of Dragonsrod, like many new authors, I thought I had a best seller on my hands and that I was going to be the next Great American novelist.
Of course that didn't happen. Despite my word of mouth and limited online promotions, I think I could count on one hand how many sales I made that year -- I gave away more books than I sold. Since then, I have sold several copies of my two book series, but not enough to let me quit the 9 to 5 scene.
Yes, you want to focus on writing that book and promoting it while envisioning all the fame and riches that could come your way.
If you are in a good financial situation, you probably can afford to write and promote your books on a full time basis. However, if you are apart of the vast majority who are on the other side of the fence, and money is tight KEEP YOUR DAY JOB! Bills need to be paid, you need to keep that roof over your head and you need to put food on the table. Write and promote when you have time!
Only after the money from your books starts to come in can you give thought to turning in that two weeks notice and pursue the writing career full time.
If/when you reach that plateau; Congratulations!