First--individuals retire, groups don't. Some high-earners do hard physical labor, most low-earners probably do not. So we're lumping a bunch of stuff into one pot. Next, maybe Mr Krugman would provide us with a graph of "return on SS premiums paid" for the upper income group and the lower income group. Don't hold your breath for that one, because we both know what it will show. Finally, in the graph above, while you see a growing disparity between income levels, I note that all groups are living longer than retirees of similar income levels in 1986, so everyone is getting a better deal than they would have in 1986. Was it "unfair" then? Is it sustainable now?
If you favor a SS retirement age graduated by income level due to differing life expectancy, you should certainly favor one based on gender (women should work longer). And race--even when decoupled from income--is a highly significant factor in life expectancy, so don't neglect a separate adjustment for that. Smokers should certainly begin getting their SS checks significantly earlier.
In some measure, "fairness" consists of establishing rules and keeping them relatively constant so people can plan their lives.