Of course, no one should invest with non-discretionary income since that would be gambling rather than investing, and yeah, discretionary income can be increased by increasing income and/or decreasing expenses.
Yeah it would seem more like gambling if one goes beyond discretionary income but some times I do have this feeling if we are only buying bitcoin with an amount can afford to loss them doesn't that make it a gambling because we are afraid we might losses them? lossing can be in many ways but the most fear in this instance is the fear of lossing it to crash or hacks/attacks. I prefer to invest into bitcoin with an amount that I can afford but not the chunks from my discretionary income, if we keep it being only a little of what we can afford to loss to me it's now a gamble which bitcoin is not.