It is still possible to invest in bitcoin without have a good cash flow as you mentioned, it's mandatory you should have good cash flow before making purchase this question is did you have discretionary income?
You don't need to be Bill Gates before investing in Bitcoin, that is the mistake the majority make and it pains them with how Bitcoin has performed. Bitcoin doesn't need you to fully adhere to your question of the discretionary income, many would use this as an excuse to dissuade themselves from investing if that is the case. Bitcoin investment mustn't always be on the platter of full comfort, investors must deny themselves some things too.
Whether you have that huge income or you scarcely have the excess, it is the decision you have to make. Are you wasting excess money on frivolities, or the wants take more than necessary from you than the necessities? That money is what you need to channel into your Bitcoin investment. If we always talk about the discretionary income as an excuse, most people will not own a single Bitcoin.
It is true that one don't need to be the richest man in the world before buying bitcoin but one need a Discretionary income to get started because from your write up you are directly saying that one can get started without having a discretionary Income that not having discretionary income shouldn't be an excuse not to invest in bitcoin, this can be misleading and could make some newbies buy bitcoin with money they can't afford to loss since they won't be buying from discretionary income, we also try to enjoy ourselves even if we are buying bitcoin.