What you said is not true, ginsan.
discretionary income is any amount of money that you still have after you pay your expenses.
Discretionary income does not need to be fixed or stable.
Sure, it is helpful to have stable and/or fixed income but it is not necessary.
If you say that a person needs fixed and/or stable income in order to buy bitcoin, you are adding an additional condition that is not necessary even if it might be helpful.
It is like saying that I need to have $100 every week to invest in bitcoin, and if I only have $50 this week or some weeks I have $10 and other weeks I have $200, and some weeks I do not have any, and the punchline is that there is a lot of variance in my discretionary income, yet I still can invest in bitcoin, even if those amounts and/or those frequencies are not known in advance.
Bitcoin is all about taking advantage to invest and be profitable at it, this may come by any chance and at any time, for us to be an investor, we may not have to depend on having a discretionary income which some also called a fixed source, instead we can have one or two sources to the income being generated, then plan on the accumulation pattern for our asset and device a strategy to use like DCA, which could best be applicable as at when we are capable enough to buy and hold as the market dips and as we are having our income flow coming.
Doing this makes it a perfect and more convenient way to invest into bitcoin and hold, by using different strategic entry points for an investment and also strategizing on the perfect timing for us to invest on the asset and hold, mostly which is when the market dumps, then we buy to sell when it pumps.
If I don't have enough money, I have to rely on one income and work only one job and some salary at the end of the week. Accumulation Bitcoin from discretionary income and meeting all responsibility the family. You may be talking about investors who have multiple sources of income or their financial situation is relatively good. I would argue for a new investor to start accumulation Bitcoin from discretionary income because they are not experienced enough. They need more time to arrange alternative income and they need more time to understand the market situation and become a more experienced investor.
The best strategy like DCA and Bitcoin accumulation would be a good stage for those investors who rely on fixed income. As you meant, if I am not mistaken, it is to accumulate regularly and have floating cash available for more buy in bearish times. However, I may agree with you if you have the attitude that instead of saving floating cash from alternative income, it is better to increase the amount of Bitcoin accumulation.