it is risky to attempt buying Bitcoin with a non discretionary fund because you can end up suffocating meeting with up with your essential needs or end up selling your Bitcoin too soon.
Buying bitcoin with anymore that is outside your discretionary income is probably buying with an amount supposed for your expenses or supposed for your emergency funds and as such it is very reasonable not to use such money because you will end up attempting to use it or spend the bitcoin at a very premature state and hence it's more like trading and not an investment.
which extra money are you talking about? people usually save a portion of their salary for the future since not everyone can save a large amount at once. after covering monthly expenses, the extra money that remains is what they save. if they invest that money using the dca method, they do not need to invest a big amount at once they can invest small amounts easily. instead of keeping that money in a bank, if they invest it in bitcoin, by the end of the year they will have accumulated bitcoin gradually. this method is good for everyone. if you reach old age without any savings, your whole life’s effort goes to waste. so, invest money that you won't need to use in the future.