i get what the author was trying to say, but i'd put it differently (as others in the thread have already indicated). i envision a few different modes of life. these modes or levels are strongly entrenched. downward mobility is more likely than upward mobility by a huge margin, but it still takes a large negative shock to generate downward mobility.

level 0: primitive accumulation of non-liquid assets. the image here is a homeless person trucking around all of their possessions in a shopping cart or perhaps someone living in a shantytown. there is no ability to pay for any kind of costs, except by the generosity of others. severe mental illness is guaranteed.

level 1: destructive equilibrium. wealth remains constant (and low) despite consistent or inconsistent income. this is where many of the working poor are -- no getting ahead, but a large potential for falling behind. a small number of fixed costs may be sustainable, but stability is very low and anxiety is very high. there is no purchase which can be made without budgeting. mental illness is highly probable.

level 2: unstable accumulation. wealth is low, but there is a slow rate of accumulation of wealth. the wealth is often dipped into during times of crisis, so on the long run, there isn't much compounding going on even if the wealth does accumulate. many purchases require budgeting, but the smallest of purchases (like a pack of gum) can escape deliberation. the rest of the working poor and most of the "middle" class fall into this category. mental illness is probable, because there is never true stability -- the amount of wealth in the bank is never enough to protect against more than one problem at a time.

level 3: stable accumulation. wealth accumulates reliably, and compounding interest makes the long-term view rather rosy. less-trivial purchases like TVs or nice dinners don't require deliberation. the real defining feature of this level is that for the first time, the means for having nice things increases over time. someone might be only able to afford one vacation a year at the start of their career, but if they stay within this level they could afford four by the end. mental illness is less likely because there are enough resources to protect against more than one shock at a time. note that major purchases like houses or cars are never beyond at least some deliberation for this level, even if other things are. this is the "upper" middle class.

level 4: accelerating accumulation. these are the upper classes. shocks are irrelevant to this level because a shock of sufficient size to threaten their well-being would probably be the end of the world or at least major chaos. at this level, budgeting does not really exist. the economic deliberations of the lower levels are replaced by a consumption deliberation. the question is no longer "can we afford this" but rather a purely hedonic "would i like to have/do this". the wealth base at this level is so large that even excessive consumption will not slow its accumulation by an amount which would require cutting down on future excessive consumption. the people at this level hold most of the world's financial resources.

is this view of things reductive? yep. the takeaway lesson is that the vast majority of the people with a positive net worth do not have genuine stability.