Buying bitcoin and working to be paid in bitcoin are the biggest sources. Signature campaigns and trading are good but they are side sources of money. Getting a job which pays you monthly is the best thing to do. However thing is that such things are not easy to find and most people dont accept bitcoin yet in the larger scale.
Hence for people who have a fiat daily job its better to stick to side earnings of bitcoin and then convert the same if needed.
Earning from signature campaign can be treated as a side earnings but there are many people who are earning a living from trading as they are full time into it so if you have a good capital to invest in trading then you can generate handsome income from trading and I think one of the best way to earn bitcoins.
There are dudes who have been trading their entire life
If you are a talented trader and Bitcoin market collapses to nothing, you will just switch to trading something else. There are quite a few options out there in this respect. Besides, best traders earn a lot of money, and I guess after a few years of successful trading they have enough funds to last for the rest of their lives even if they are not going to trade anymore or just switch to less risky and more satisfying endeavors (e.g. writing books about how to become a successful trader)