You are very correct without a sources of income it can't be possible to invest in bitcoin, and not only that if a person is living in paycheck to paycheck it will also be impossible to invest in bitcoin, to invest in bitcoin one need to have sources of income to buy bitcoin consistently with their discretionary, and one also need a sources of income to set up emergency fund, for that is the only wey you can run a smooth bitcoin investment.
You were saying something nice until you ruin it by making that statement i highlighted in your text there. It is very possible for someone with paychecks to invest in Bitcoin as long as they pay their expenses and have some money left. TBH it is all about consistency not amount.
What is the point of dca when you can't invest with your paychecks. Let me tell you how people do invest with a paycheck through dca if you have someone like that. First when you receive your paycheck weekly or biweekly it is not compulsory to take out some percentage to invest immediately because of cost for covering fees. You can take out your expenses from your paycheck while you leave the rest till the end of the money then you calculate what is left as your discretionary income from your paycheck. That way you can invest though dca monthly after you have gathered what's left from your paycheck at the end of the month.
Have you asked yourself how those who dont received fixed salary but are into contracts job get to invest in Bitcoin? You don't necessarily need to have a monthly income before you can invest.
You're right about the emphasis you laid on that highlight but still deviated along the texts. In the principles of saving and investments, you don't save what is left after expenses, you spend what is left after saving. Talking about summing up your leftover funds off your paycheck may also never allow anyone invest in bitcoin because there may never be any part of the fund left to at the end.
In my own order and DCA approach, I advocate planning. Before your paycheck comes in, you already have idea of what it would be. The best is setting up your expenditure based on your NEEDS, setting aside a percentage as emergency fund and then ripping off a particular percentage as DCA which you can invest a part with. The problem is that most people always believe that their earnings is not enough which I think @Joy-maker may also belong to believe that paycheck earners don't invest in bitcoin. Investing in bitcoin is intentional and no amount is ever enough for anyone until you decide to restrict your luxuries. Invest by DCA, no matter how little. Paycheck earners and salary earners somehow has better chances of planning because they are always sure about when their income is coming and what amount is coming and can actually plan ahead.