I doubt anyone is saying that a steady income is not important, but instead saying that a steady income is not required in order to determine to invest into bitcoin. The thing that is needed is discretionary funds, and discretionary funds can come from a variety of places, including from non steady income sources, or even prior building up of back up funds or even from the use of debt... and yeah, even though a steady income can make things easier, it is not required in order to be able to buy bitcoin.. .what is needed is discretionary funds.
Many people believe that you can only set up a discretionary fund if you have a steady source of income, however stable income is not necessary before investing in Bitcoin. The main thing is to have some extra cash that you don't need to cover your other needs, this money can come from everywhere like side jobs, savings, or even some money you've saved for a long time, and there's no need to do this with your monthly salary.
Of course, if you have a steady income makes it easier to continue investing and building your Bitcoin over time with a rest of mind, but it is not necessary if you understand how Bitcoin works. And as long as you have spare money and you can afford to lose it without affecting your other expenses, you can invest in Bitcoin. What matters is that you use funds that you are willing to risks rather than relying on your monthly salary.
If you are investing into bitcoin rather than trading, then you should realize that any of the money that you use to buy bitcoin is getting locked away for 4-10 years or longer, so you don't need the money.
We invest with an expectation that in the future our investment will pay off and that we have chances to be better off for investing rather than if we had not, even though at the same time, we should recognize and appreciate that our investment might not pay off and could go to zero. We should weigh our considerations in determining our position size, and yeah, if our income is not certain or steady, then we may well have to be more cautious in regards to how much we are able to invest and how steady we can invest into bitcoin, so there surely are advantages to having a steady income or figuring out ways to make our income more steady, even though we don't have to have a steady income for each time that we are considering to buy bitcoin and considering how much, then we are likely considering how much money we have that we are ready, willing and/or able to lock up for 4-10 years or more.
I understand your point, and I agree that if you invest in Bitcoin for the long term, you should be able to leave the money for many years, perhaps between 4 and 10 years or even more. That is why it is important to be sure that the money that you invest will not be used for daily expenses or emergencies.
However, before investing, everyone should have a positive mindset toward their investment. And you should decide how much you can invest in Bitcoin, either weekly or monthly without touching it, and this should be extra money, not money for other needs. If you invest money that you might need tomorrow or next month, you may be forced to sell at a difficult time, especially if the market price falls.
Sure. A steady income makes it easier to invest weekly or monthly, but even if your income is not steady, you can still invest as long as you are serious with it. Since we have to maintain our investments and separate them from the money that will cover our other needs. This allows you not to fear and hold your investment for the long term without stress.