Let me make it clear that without a stable source of income it is impossible to succeed in long term Bitcoin investment even if you have a very strong backup funds it will run dry as time goes on and before you no it you will start dipping hands into your Bitcoin investment, nothing can save your bitcoin investment if you don't have a stable income, and just like you said one needs to have a stable source of income before starting his or her Bitcoin investment journey because if you start bitcoin investment without having a source of income and then hoping to find one when you have already started then you are taking a very big risk and is 50/50 you may succeed and you may not so before you start reason all this.
a normal person should not even think about investing with out a stable source of income because after taking a hit for the market the person might lose his mind because that moment he might have taken a big risk by putting all his funds ,and just imagine that happens the person will become forstrated and even start tell other people that bit coin is a scam and that is wrong so it will be more safer to have a stable source of income