Discretionary income is not even money that you can afford to lose. In my estimation, it is money that is waiting to be spent on something that is not yet. Known. It is not even emergency funds. But I see the role it plays as a means for managing risks because you already know that if you use it to invest I'm Bitcoin you may lose it or not. Therefore your mind is already made up on either of the outcomes. And if you like, you can even use this discretionary income for your DCA for Bitcoin.
Yes discretionary income is not an amount you can afford to lose it is usually the money left for savings and spending after taking care of the essential needs. So I kind off agree that the idea or narrative for investing into bitcoin should be change to investing your discretionary income rather than investing what you can afford to lose. Instead of investing or spending on other less important things you can simply just have them on bitcoin which has proven to actually be a good asset and store of value
if you invest extra money, its more likely that you won't touch it for the near future, lots of people invest money then after a week or a month they withdraw it since they needed money, that is not a good idea, investing money that you will think more often, should not be invested, because, you will lose money that way, but if you use those extra money there is a good chance of a profit, in the future.
That money then shouldn’t be regarded as discretionary income because it is something you need, discretionary income is after even taking out emergency funds too plus all necessities are covered. It is very bad to even invest into bitcoin with an amount you can even be in need in less than a year, sometimes I recommend a time frame of 4 years the limited time you can be able to hold that money