We have been told many times to invest only funds which you can afford to lose in bitcoins. But the question is that most of us do not have that extra funds to invest. So, what we do is we buy bitcoins from our savings, and from whatever money we get, we try to buy bitcoins.
With this reason that's why investing is not for everybody, precisely investment into risk asset like Bitcoin is not really for everyone if you can't stick to the rules of the game involved with investment.
Secondly, If you said middle class don't have money that they can afford to lose, Then I think you really have to change your view on the way you go about investment, because this is why people end up hurting theirselves investing their life savings in a volatile crypto market and end up loosing all. Therefore, it's much better you take a little part of that savings to invest than using all.
Here's is an example of something that could help you,
Some people spent a lot of money unecessoryly on drinks (alcoholic drinks) or other unecessory things, while this kind of money could have been saved gradually and put into some investment like buying Bitcoin and you won't regret whatsoever thing that may happen to it in the end, so these are also kind of money you can afford to lose.
The Bottom line here is from my point of view, money you can afford to lose means that they're pre-informing you about an investment risk warning that you're likely going to lose whatever money your are coming into invest, and so for safety, should ensure that it's always the money you can afford to lose so you don't have to blame anybody in the end.