If we end up spending our income prior to accounting for expenses, then when we have expenses that are due, we either have to spend from our back up funds or we have to spend from our bitcoin, and so we are trying to prevent situations in which we will have to spend from our bitcoin at a time that is not completely of our own choosing, and so we sometimes could make mistakes which provides greater justification to have various back up funds.
This happened before, I sold my some bitcoin when I was in trouble. Although I thought there was no bigger problem than our trouble then. I sold my some bitcoin, spent my bad time in daily life, and got rid of the danger. However, I agree with you that we must have a backup fund in addition to investing in bitcoin.
Although Bitcoin investment also works as a backup fund for us, if we have another fund, we will not have to sell bitcoin anymore and will be able to continue investing for a long time.
Your bitcoin investment should never serve as a source of backup fund for you, it is a bad practice to see it as such. You should build out your backup funds to protect your investment from being tampered instead of seeing your investment which should be held for a long term for future profits as a backup fund.