We might have differing definitions in regards to what is mandatory and what is discretionary. Many times we might want to live similar lifestyles as our neighbors, yet there still may be things that we have discretion over, whether we buy beans, hotdogs, chicken, pork or steak. I doubt we will necessarily agree which expenses are mandatory and which are discretionary, even though surely guys might even lie to themselves if they consider certain kinds of expenses to be mandatory when they are discretionary.. yet let's say that a person wants to have a flower business, and maybe there is delivery involved in the business, he might have discretion regarding how to deliver and whether he owns a vehicle, rents it or contracts it out and he might even have discretion whether to have a bicycle, or a motorcycle or a car or a van..and he might even have discretion whether to buy the 2002 model, or the 2012 model or the 2025 model.
You are right, for me I think In other to fine out the expenses that are mandatory and discretionary in our daily life is by checking our income just like the example you gave one may become confuse as which to buy but your income will help you determine that, like the example you gave if your income can only get you a bicycle I don't think you will be contemplating again on buying a car or a motorcycle however the only time you may be contemplating is when your income can afford you all that you have mentioned and at this point one can get confuse on which expenses are mandatory and which are discretionary so meaning the more richer you become the more confuse you will be in knowing which expenses that are mandatory and are discretionary.
I don't think that there is anything too difficult about knowing which item is mandatory for you to get and which is not. From elementary knowledge of finance and economics, the concept of
wants, needs, and opportunity cost can easily be applied here and you can easily identify which item falls under what you just want, and which falls under what you need knowing fully well that your wants are insatiable in the first place. you can always push your wants to the future when you will be financially strong enough to get them while you meet your needs since they are part of what keeps you going as a person.
again, note that we can easily adapt to things. if you are earning $100, for instance, you can easily make your budgeting to suit that $100, and it can carry you for a month if that is the only thing you are getting. if your income increases to $500, you might be surprised that if you don't do proper budgeting, the $500 can still finish in the same manner that the $100 will finish. what that tells you is that while how much you are earning helps you in some way, it is proper planning and budgeting that helps you effectively utilize your money for what matters most.
i still think that allocating a percentage of your income for your investment and leaving the rest for your expenses is the best way to deal with the issue of how much you will be willing to make use of as your discretional income or not. if I have decided that regardless of what i am earning, that 15% or 20% is compulsorily going into my Bitcoin investment. once my income enters, I will just automatically take out the 15%, and whatever is left can then go into all my wants or needs. assuming after taking out the 15% what is left is just $160, what that means is that the $160 is what I am currently earning. the allocation for my Bitcoin investment can be treated as me sowing a seed into the future that is to be treated as separate from every other income. if you are desciplined enough, there is no situation you cannot adapt to.