I agree with this, if there is a plan and discipline, long-term investment is possible and can be sustainable even with fixed income. If someone's monthly income is $500, he can invest at least $20-$30 after covering all expenses. In the DCA method, no matter what the price, if he continues to invest a certain amount regularly for five years, it will turn into a large number. Which is possible only through fixed income. There are many investors who have started with small amounts and have been successful consistently. The key to investing is discipline and consistency. If you start with a very small portion of fixed income, it will turn into a significant asset over time.
I don't get the point that you said fixed income or are you trying to say a stable source of income. A fixed or stable source of income isn't a necessity for a no coiner to start his bitcoin investment, provided he has a discretionary income that he can use to buy immediately, even though he cannot be consistent with his ongoing accumulation. Some persons might don't have a good source of income to provide them regular discretionary income every week/month. If they have started their bitcoin investment, they can look for means to increase their income through looking for a second source of income, learning a skill or improve their educational qualification to earn higher. When that is achieved, you now have the flexible to keep your bitcoin accumulation ongoing with weekly DCA consistently and persistently for 4-10 years and above.
I wouldn't agree with you that a fixed income is what one needs to be consistent in his DCA accumulation overtime because his income is not increasing, and the rate of inflation is getting higher overtime killing the value of fiat slowly overtime and the price of bitcoin increases overtime. If I am receiving $300 weekly, and my monthly basic needs and expenses is $230, my discretionary income is only $70. If I don't look for a way to increase my income from now till 5 years' time that $300 will not be enough for me again due to inflation and the high cost of things. I might have just $10, or nothing left as my discretionary income and if I continue like that on the 7th year, I might not have any discretionary income to invest in bitcoin and i will stop my bitcoin accumulation journey.
This is the reason why as you are investing in bitcoin, you must look for other means to improve your income to keep on your accumulation journey ongoing and also increase the amount that you are using to DCA due to an additional discretionary income.