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.
It's the beauty of Bitcoin that we can continue with any amount. If in a particular week I don't have any money or less money compared to what I am investing then I can compensate that in future when I have additional money. The key point is never stop investing in Bitcoin specially those of us who are new and don't have enough Bitcoins. The results of continuous investing into Bitcoin will be seen after 5 or more years and then one can feel proud of his decision of investing in Bitcoin. In start it may look boring but after 5 or more years it will be full of excitement.