I'd be curious about how success is defined here. Career is a pretty narrow lens to define it by so I'd hope for something more expansive.
Many of the benefits of having friends in high places are outside of traditional career ladder. The expedited (insert annoying process here), the vacation you're invited on, the unintentional influence you have on some big thing because you happen to be an ear to the decider, etc etc.
There's been a few studies. [1] is one framed in economics, so it measures earnings. You're right, though, that earnings is probably an overly blunt measure of success at best. I think the difficulty in measuring quality of life statistics is that much of it is difficult to quantify, so studies fall back to easily quantifiable metrics.
Edit: [2] expands the measures to include educational attainment and family outcomes. Reference [3] relates to socio-economic class, while [1] relates to race/ethnicity. [3] was the one I had in mind during my original comment.
[1] Krueger, A., 2012. Estimating the Effects of College Characteristics over the Career Using Administrative Earnings Data Stacy Dale Mathematica Policy Research.
[2] Ge, S., Isaac, E. and Miller, A., 2022. Elite schools and opting in: Effects of college selectivity on career and family outcomes. Journal of Labor Economics, 40(S1), pp.S383-S427.
[3] Dale, S.B. and Krueger, A.B., 2002. Estimating the payoff to attending a more selective college: An application of selection on observables and unobservables. The Quarterly Journal of Economics, 117(4), pp.1491-1527
Earnings is probably a reasonable proxy for something like a business school. I'm not sure it's great for a liberal arts college. Way back when, I looked at some of this stuff and you're right that figuring out what outcome(s) to fit to is challenging. Undergrad GPA is pretty clearly not what you want either but generalized career or life success is pretty hard to quantify.
I'd be curious about how success is defined here. Career is a pretty narrow lens to define it by so I'd hope for something more expansive.
Many of the benefits of having friends in high places are outside of traditional career ladder. The expedited (insert annoying process here), the vacation you're invited on, the unintentional influence you have on some big thing because you happen to be an ear to the decider, etc etc.