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For investment sustainability, a fixed income is needed, which is called discretionary income.
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.
Having a fixed income to invest in Bitcoin is just like a policy, and as Bitcoin investment is meant only for those who have a fixed income. Having a fixed income is not necessary to invest in Bitcoin; it is better to say one needs an income or source of income. It is not essential that this must be a fixed income, but the most important thing is having a source of income that you can depend on at all times. Then, invest the amount you can afford in Bitcoin, and this must be consistent to accumulate Bitcoin for the future.
There are some people whose income is not fixed, especially the self-employed. This does not mean they need to have a fixed income before investing in Bitcoin; as long as there is a source of income, anybody can invest in Bitcoin.