There are loads of Harvard kids who make a smooth transition after graduation to a high-paying job in investment banking or consulting. Many of them have wealthy/influential parents who call in favors from their friends to make introductions and help get the right internships for Junior after sophomore and junior year, building his/her resume and clearing the path to an offer following graduation. Many other kids from comfy backgrounds have the connections and social capital to get other kinds of prestigious jobs in media, advertising, entertainment, tech (yes, even tech), etc.
That doesn’t mean, though, that kids with other skills who don’t enjoy those advantages won’t do well; after all, a disproportionate number of these opportunities are offered to students at Harvard and its peer schools. Goldman Sachs and its ilk recruit a lot of kids from Harvard and similar schools, and they don’t all belong to the same country clubs. Many came to Harvard without prior connections, distinguished themselves on campus, did a lot of legwork and forged their own path to Wall Street. So there are more kids with a leg up at Harvard, but there are also more opportunities specifically available to Harvard students - and if someone is the sort of kid who gets into Harvard without certain advantages that others enjoy, they may be well-placed to exploit those opportunities.
Legacy by itself gets you two things: (i) a second look, which is a guarantee that you won’t fall through the cracks and be denied after some junior adcom spends five minutes skimming your app (and in the context of >40k apps, this is meaningful); and (ii) something that functions as a modest tiebreaker, in a game where many applicants are fundamentally interchangeable and there are consequently a lot of ties.
That said, there are legacies and there are LEGACIES. An example of the former is someone with a parent who went to Harvard and is now a mid-level professional in a small city, who never donated significant amounts of time or money and isn’t in a position to be useful to Harvard and its affiliates in any meaningful way. In that scenario, the fact that the parent went to Harvard is modestly interesting, nothing more. The admissions decision is really almost all about what the kid themself brings to Harvard; the legacy status just ensures that the kid’s app will be appropriately considered.
An example of a LEGACY is a kid with two parents who went to Harvard, at least one of whom is now prominent in business, government, Hollywood or some community/group that’s highly relevant to Harvard, who is in a position to be useful to Harvard and its affiliates and likely has given substantial and growing amounts of time and/or money to Harvard, with indications of capacity for a lot more in the future. That family matters a lot to Harvard and, as you might reasonably assume, the bar for the kid is much lower, with zero chance that that application doesn’t get significant focus at the admissions office, possibly bolstered by a call from the development office.
[Not that there’s anything wrong with that - Harvard is a private university that can admit who it likes, on the basis of who it feels are most useful to it - but that’s a topic for another discussion.]
The number of legacies admitted is naturally limited by the need to meet other institutional priorities; it’s good for Harvard if they can kill multiple birds with one stone and find, for example, a star quarterback who’s also a wealthy legacy, an URM and from Wyoming, but this is unlikely. I’ve often thought that Harvard and places like it need to admit enough legacies for the alumni to believe that legacy status provides a real boost, so that they’ll play hard for the Harvard team in the hope that it helps Junior’s admissions prospects, but not so many that Harvard has to skimp on quality and turn down too many kids that it really wants or needs for other reasons.