It is a simple consideration that an investor should have alternative sources of income and also if you have extra money to invest then Bitcoin would be the best venture for you. Your objective is to hold Bitcoin and your effort to be long term is DCA or single investment. Bitcoin is only recommended for newbies who do not bear additional risk in DCA strategy. The review of discretionary income source that you mentioned is that it is a long term investment endeavor where an investor is looking for a great strategy for his future.
I agree with you if you want to be able to carry out investments well on Bitcoin then we must have some sources of income to be able to maintain the assets in the long run because if the income we have hanay enough for needs is certainly not possible to run the investment because it takes a time that is not a short time to produce profits from the investment.
When someone wants to survive in the long run, it will certainly be very good in using the DCA method because they can always buy consistently according to their abilities after fulfilling all the needs they need because it will be very unlikely to survive if the necessary needs have not been fulfilled properly.