If you are accumulating Bitcoins and want to hold them for long term then best approach is to accumulate them through discretionary income. While your job salary or business income can be used to meet daily requirements. Why discretionary income is important is because it allows to dormant your Bitcoin for longer duration. Without discretionary income or if you are accumulating bitcoins with your salary then you might cash out your bitcoin in the long run, which completely destroys your plan of long term investment.
Seems you don't really know were discretionary income come from or how it is gotten. You are missing something here. It is from your salary or profit from your business that you get your discretionary income from. Discretionary income is the money that remains after you have removed money for your day to day expenditures From your salary or from the profits from your business. In nutshell it's from your salary or profit from your business that you can get your discretionary income from. So discretionary income is not another money gotten outside your salary or profit from business.