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.
If the person only invests in bitcoin, it would be unwise to only put 10 of his total 90 dollars of discretionary income. Because although this is certainly the right of each investor, it feels very half-hearted. Because by having a discretionary income of 90 dollars, if all of it is bought into bitcoin, of course in a period of 10 years or more it could be very large. But if for example only investing 10 dollars every week, it is certain that in the next 10 years the person's bitcoin portfolio will definitely not be as big as if investing 90 dollars. So if for example you are not yet able to invest all your discretionary money in bitcoin, I think investing 85% of your total discretionary income could be a good option. Because 85% can certainly be said to be quite large, and besides that, there is still 15% of your discretionary income left. But I personally prefer to invest more than 95% of my discretionary income in bitcoin. And I use the remaining 5% as an emergency fund or for other things.