Honestly in the right sense, Someone that doesn't have a Discretional income shouldn't have any business with Bitcoin investment because what's is needed to accumulate bitcoin is from his discretional income. Sometimes when he gets too excited investing in Bitcoin, he may start buying bitcoin outside of his Discretional income by using funds meant for his basics needs to buy Bitcoin regularly.
This is where good cashflow management practices and proper discipline take pre eminence. With good cashflow management practice, you'll carve out your expenses and separate it from your discretionary income and further divide your discretionary income into funds for Bitcoin accumulation and those for building backup funds.
It's important to note that buying Bitcoin with funds that are not your discretionary funds can be described as being over aggressive and it's not a good Bitcoin investment practice and furthermore it's not sustainable in the long-run.
I largely agree to what you said here with the exception of the bold words in your statement because in my own understanding, any investment made in Bitcoin outside your discretionary income is pure gambling, because it will be just be a matter of time before he tempered with his or her bitcoin investment because the money used in investing in bitcoin is meant to sort out one of his basic needs, so the right word you should be using is
pure gambling not over aggressiveness
, because it can never be sustained on the longer run.