I disagree that lost income is purely psychological. There are biological realities to deal with, especially in the US without the social safety net of other countries, that make lost income important. The first two that come to mind are the significant cost of raising children and retirement. Those are ticking clock issues to many people.
I agree they are two different worlds for low and middle income people. But we could play this game with even lower income people elsewhere in the world and reduce the issues of the developed countries poor.
I think we probably agree on the situation but disagree on what terms make sense to describe it.
For example, if you get your bachelors in 3 years instead of 4 and spend $30k on tuition and earn $90k by working, then that would result in an extra $120k in compared to spending 4 years in college.
I class it as an expense, because everything has an opportunity cost, which is defined as your 2nd best option. It is not always useful, for example thinking about spending time with your kids rather earning $15 driving for uber or something. But it could be, if you need that $15 to feed your kids.
The concrete part is that people with that level of income (large, but not so large as to make all problems disappear) know that to be comfortable in the future they need to save a big chunk of their income. The people living with much lower incomes just don't have that luxury at all. Which is kind of the point - it's totally different worlds.