I don't think there is anyone out there who doesn't mind losing money no matter how small, before I decided to invest, it was to grow my money, not lose it, also discretionary income isn't money you can afford to lose, it's what's left of your income after all your basic necessities have been taken care of so it's definitely not money you can afford to lose, it's just money you can use at your discretion for whatever you want to use it for, if you are robbed of your discretionary income you will be pained by that loss regardless of how rich you are because the richer you are the higher your discretionary income is, losing it will hurt I don't think any investor want to lose their money.
In my opinion, an investor who knows the essentials and is well-versed in Bitcoin will never decide to invest an amount of money that he cannot afford to lose.
Using the term
money that he cannot afford to lose makes it sound alot like gambling and not investing because gamblers are the ones usually adviced to gamble responsibly and with money they can afford to lose. An investor should never have this mindset
In as much as I understood your point I will also want you to understand that for the very fact that Bitcoin is a volatile asset and we believe it has potential over a long term, it is a reality that it is not 100 percent guarantee therefore, considering our discretionary income as a money we can afford to lose gives us a balance sense of reasoning that we could lose whatever we are putting in even though the chances is very low, considering our discretionary income as money we can afford to lose reduces the chances of getting overly worried away about losses and increase our willingness to invest even more in Bitcoin, no doubt that such statement is mostly used in the gambling section of which Bitcoin is nothing of such but however, we can still relate or tell the difference, Bitcoin is not a quick and get rich scheme and the reason why no one should be gambling with it, but anything about gambling only relate to getting quick money so even if such statement is use in Bitcoin we should be able to know the difference in context and the risk involvement.
People who put what they can not afford to lose or let go are typically traders because they have the intention to make millions if the market move according to there speculation but funny enough they are not the one controlling the market and not only traders does this some people who don't have a better knowledge of what Bitcoin is all about do make the same mistake and when they do that and along the line they found out they are stranded they will be tempted to touch there investment because they didn't calculate, plan before investing and I don't know why people are rushing to invest in Bitcoin when they can not maintain it, starting is a not always a problem but the problem is maintenance, you can not buy a car and then refuse to be maintaining and servicing it because definitely it will spoil that is also how Bitcoin investment work.
Ie agree that Bitcoin is volatile and is not 100% risk free then I will consider such phrase invest an amount we can afford to lose as a way saying that we should only invest an amount that suits our individual risk tolerance level such that in the case of lost it will not negatively affect us emotionally and financially and in other areas of our life.