Discretionary income is not meant to be spent on things that are not meaningful; it is the money left over that needs to be spent wisely. As a reasonable investor, it doesn't make sense to go clubbing when you can't afford it, and there are other tasks that are valuable to meet. Generally, money is meant to be spent, but if you spend it on things that have no meaning and no value, then you will fall back to square one. With discretionary income, you need to comport yourself very well and know how much you need to invest in Bitcoin and what will be left with you.
You are misunderstanding everything. Your discretionary income can be spent on anything you want. The discretionary income of everyone differs, my discretionary income may be higher than your monthly income so I can spend on any other things I want to except investment. It doesn't have to be for investment only. I only invest 50% of my discretionary income in investment and the reason why is because I also have an ongoing project which I am running and I need some funds for it. If you dont understand what discretionary income means then go back and read it again. It is not defined as a money meant for investment alone, but the money left after taking care of our expenses, emergency funds and savings.