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.
The primary purpose of using an amount that you can afford to lose to invest is that you wouldn't feel much of the effect if your investment doesn't goes as planned.
Truly Bitcoin investment is not gambling, but due to the fact that we live in a world full of uncertainty, you can't just risk it all on Bitcoin which you aren't sure that may play out as planned in the future, even though it potential is very huge, you just have to manage your risk properly to be on the safer side, and you do that by investing only what you can afford to lose, not more than your financial capabilities.