> Nassim Taleb makes some good points about that here[2], for instance, ~ 70% of Americans will spend a year in the top 20% and only ten percent of the wealthiest five hundred American people or dynasties were so thirty years ago.
In this specific instance wealth vs income matters because the above statement is true of income, but very untrue of wealth.
What makes you think it's untrue of wealth? 80th percentile wealth in the US is a net worth of ~$500k. That's where most people are at retirement when your house is basically paid off and there is money in your 401(k).
You're assuming "retirement" is still at 65, but increasingly it isn't. The median is actually highest at the oldest age group on that chart.
Everybody also forgets to include the net present value of social security (basically an annuity you were forced to buy), which is a disproportionately large amount of the net worth for lower income people both because they don't have as many other assets and because the income cap limits how much it adds to the net worth of the people at the top. (Though people would generally have a higher net worth without it; it pays back less than you'd have from investing the same money in an index fund.)
In this specific instance wealth vs income matters because the above statement is true of income, but very untrue of wealth.