I mean, we should only use leftover income after all basic needs and emergency funds are covered. Because this way we can build assets solidly and calmly without pressure. Profit is indeed the ultimate goal we hope for but don’t let it dominate our thoughts.
While I agree with your statement that you should not use the money needed to buy Bitcoin to buy it, I do not agree that you should start buying Bitcoin only after the money needed for the emergency fund is fully accumulated. As you know, not everyone's income and ability are equal. If a person has a small amount of discretionary income every month after meeting all his expenses, then it may take him a year or more to build an emergency fund equal to a few months of expenses. If he is busy all this time just building an emergency fund and does not start buying Bitcoin despite having the ability, then he will probably be behind in the long-term accumulation opportunity. I am not saying to ignore the emergency fund and invest all his money in Bitcoin. If he wants to, he can simultaneously consider his situation and invest a part of his emergency fund.
There is nothing that should happen that should make us to invest in bitcoin with our emergency funds, that's a wrong step for an investor to do, our emergency funds is there to protect our bitcoin investment not to be sold when we haven't reach our bitcoin over accumulation point because when emergency come and there is no emergency funds the next thought will be to sell off.
Bitcoin investment should be done using our discretionary income that is the rightful funds to be use when buying bitcoin, any one using their emergency funds to buy bitcoin is just gambling with their investment as they are indirectly trading with their bitcoin investment.