I am not assuming anything, except that anyone investing into bitcoin has to have discretionary income if such persons are going to invest into bitcoin or anything else. If such persons do not have discretionary income then they are gambling rather than investing, and I am not suggesting that anyone gambles.
This is what is happening now sir.. 
Sure, a lot of people make mistakes at various times because they might be investing beyond their discretionary income, so then they may well get frustrated when the BTC price drops because they were hoping it to go up so that they could scalp off some profits.. which surely some times those techniques can work in favor of the person who is gambling, but surely it does not seem to be a good way to approach such a volatile asset like bitcoin tends to be.
Just on some other thread someone was angry about the fact that they said its better to invest with money that you can afford to lose or disposable income and this also came to heart, it's possible that must people that invest more than their disposable income normally have some high hopes for bitcoin and just invest some huge amount and they are not even ready for the reality that bitcoin is volatile and can also go low.
I believe that this advice is good to let people know that they is possibility to encounter loss in your bitcoin investment and its good to invest with what you may feel okay to lose, and moreover your investing long term that means you have enough time to accumulate bitcoin and using DCA you can just set an amount to invest on a regular basis.
This advice is not only useful for volatile assets like bitcoin but also for all other investments because everything has risk and we should not be too subjective. But honestly, I don't think many people would follow this advice even though they already know it by heart. Because everyone wants high returns and everyone wants to get rich quickly, I guess most people are still investing money they can't afford to lose.
Additionally, it is true that bitcoin is high risk due to its volatility, but I believe that as long as they are not investing with borrowed money, and can hold for the long term, taking on a little more risk is not is a very worrying thing. The most worrying thing is that many people are investing blindly with borrowed money, because in the long run bitcoin will definitely increase in price but no one knows exactly when.