I agree with you here. $90 and $10 man such investor will be seen to be an unserious investor or a greedy one. Only an investor who is not ready to invest properly into Bitcoin will invest only $10 out of the $90 discretionary income. Unless he has other places he is investing into it. Maybe he is diversifying his portfolio so he only choose to invest 9% of his discretionary income in Bitcoin but that is not an ideal way to invest.
For newbies that just started its either they should focus on Bitcoin on the early stage rather than diversifying into several asset with the same frequency they are using to DCA in Bitcoin. After they must have had a good amount of Bitcoin in their portfolio they can choose to diversify. Diversification should come only when they are properly settled with the first Bitcoin investment goal if not they will only end up having little amount of Bitcoin in their portfolio. Meanwhile on the contrary they are jeopardizing their investment gradually thinking they are investing the right way.
I disagree with you, if someone is earning $90 a week or a month and then the person decide to use $10 to be accumulating it is not wrong reason because you don't know what the person is taking care of and before he decided to use $10 it is possible that it is the only money he or she can spare or let go as a matter of fact the $10 is even more than 10% of $90 so it is not wrong rather Bitcoin investment most times is based on capacity how much you can handle your investment so that you don't run into problem.
I join you in disagreeing with him, you see, some people think based on their personality and responsibilities without carefully learning why the person who decided to use $10 out of $90 decided that. That money is also big and sustaining in some countries, the exchange rate has to be considered too.
Besides, what can the person who keeps $80 to himself but invests $10 in Bitcoin possibly lose or suffer just for the shortfall of the $10? I call this a commitment to investment and dedication to Bitcoin. The good part is that it will help fight unnecessary spending on wants and frivolities, and most importantly help the person secure a better finance in the future.