Getting to the point where you can give up your day job to become a full-time writer takes time...sometimes a lot of it.
Alastair Reynolds and Terry Pratchett already had several published novels under their belt before they could give up their day jobs and concentrate fully on writing.
Terry Pratchett had worked as a journalist and then as a press officer the Central Electricity Generating Board until 1987 when he became a full time writer. That was after the publication of the fourth discworld novel, Mort, and several other novels in the 70s.
Not exactly an overnight success story then.
Alastair Reynolds worked for the European Space Agency until 2004, and only took up writing full time after the success of four novels and several short stories.
In short, and this is the thought for today, financial success comes only after perseverence and may take time. The occasional instant success story is dwarfed by the long drawn out successes of people like Pratchett an Reynolds and the more common tales of struggle. If you're into writing for a quick buck...forget it.