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.