By definition, you need to have either income or some side amount of savings (or other investment) that is greater than your expenses in order to be able to invest. If you do not have excess, then you are gambling rather than investing.
Of course, you could also invest if you don't have extra income but you are expecting your income to increase, but that would be a bit risky also, or you can invest based on future income that would be considered a loan, which is also risky and problematic if you don't already have funds to be able to service the loan and strong chances that your future income is not going to dry up.
If you are using the anticipation of paying the loan from the appreciation of the asset that you are buying, then you are gambling.. which may well be ok. if you have back up funds to service the loan, otherwise if you don't then you are likely taking excessive risk, which is also gambling. Sure people can do whatever they want, and sometimes gambling techniques work, but each of us should be careful if we are employing techniques that might be difficult to recover from if the price were to move against us or other unexpected things happen to cause us to lose our principle.
Of course, passive income will launch a DCA investment process in accumulating Bitcoin. Many people in the world may not have a permanent job but they want to invest and maybe it is a long process for them to have a certain amount of BTC. Yes, we encourage anyone who is confident in investing in Bitcoin to do it using DCA because they need to arrange their entry in such a way to find the lowest price. So back to the issue of income, investment can run smoothly if someone earns passive income every month so he is not burdened by the circumstances of the investment he makes.
I am not sure if you are using the term "passive income" differently from how it would usually be used. Generally speaking passive income is meant to be a kind of income that a guy would receive from investments, meaning that he does not have to do much if any work for the income, and sure passive income might be achieved based on past work that the guy did and then he invested money and was later able to live off of the income earned from the investments.
Sometimes there might be disagreement in regards to how little work someone might need to do in order for the income to qualify as passive income, so for example, his merely keeping track of his income (bookkeeping) probably would not be counted as having to work for the income, but maybe if he bought property, but he is managing the property, then the rents might be quasi-passive, depending on how much work he is having to do in order for the income to keep coming in.
Apart from that, Bitcoin has become an investment that is attracting quite a lot of attention at the moment. I noticed a lot of newbies keep coming in to buy bitcoins and hold them. Yes it's an extraordinary process of their desire to achieve success in bitcoin investment. However, from that perspective, we certainly don't know their principles in balancing their financial cash, but of course they have arranged it as best as possible so that the Bitcoin accumulation target can run smoothly according to their planning.
If a person might have between $800 and $2k per month of income (maybe most commonly a income of $1,200) and $1,000 worth of monthly expenses, then his variance of income may well cause him to keep an extra float, so that he always has enough for his monthly expenses, even during months that his income is less than his expenses. His investment would come from the discretionary/disposable income which seem to usually be $200 per month or $50 per week, and yeah he could max it out on buying bitcoin, or he could use some other strategies, but if he already has 6 month of emergency funds (maybe $6k), and maybe another 3-6 months of float and/or reserves, he might have room to be more flexible including perhaps sometimes dipping into his float and/or reserves to buy extra bitcoin and/or perhaps other times deciding to allow his float and/or reserves to build up to higher levels.