Using bitcoins as a sole and single source of funds as a full time job is difficult, I don't even know if traders can earn enough to only rely on bitcoins, the other problem is volatility which makes making stable profit from bitcoins a very difficult task. The only ones making full-time work on BTC are developers and employee of large bitcoin businesses, each of these have staffs even exceeding a small IT company.
Good traders are able to rely on the price variations of bitcoin and other altcoins, the movements are huge, if you can time it correctly you can easily make your living just by trading, it is not easy to do as you may guess but for those that are able the skill of trading will change their live for the better, and this is even more pronounced when you can supplement your income through other income streams.