I guess it depends whether you think Oxbridge should primarily be a means of social mobility, or whether its priority should be to nurture the best minds in their subject and advance academic research.
Schools exist for the purpose of the students, but by the time you get to employment it's about objectively finding the best candidate. University falls in between, and different opinions about its primary aim are valid.
I've certainly seen it argued that an Oxbridge education has more social benefit if given to a less able but less privileged student than when given to a more able but more privileged student, since it makes more difference to their life outcome. But that can only be done for a minority of students without degrading the value of an Oxbridge education.
Anyway, I think it's wise to be clear about what Oxbridge should be trying to achieve (and what the cost of that choice is: there is a cost either way).
If you believe the aim should be for Universities to take the candidate who will be the best physicist/doctor/linguist by the time they finish their degree, then the tricky thing for admissions tutors is to compare two candidates current potential (not genetic potential at birth, which is pretty irrelevant really), when they have had very different educational experiences.
I agree with@Circe7 that education changes a person and makes them better than they would have been. It's not fake. It may be privilege which gave the person that education and those experiences, but they are now part of the person and contribute to that person's current potential.
It's quite fixed-mindset (and anti-education!) to think otherwise, surely?
Do you really only see Oxbridge as a golden ticket to a better job, rather than as an outstanding educational experience which will form a person's mind and outlook? Why do you think 14 years of school is different?
I'm not sure Cambridge is getting it quite right just now, given the high difference in outcomes based on school type. I find it interesting that when it was state school students getting better degree results, that was readily accepted as evidence of a barrier to entry from State school. But now that the pendulum has swung the other way, it isn't accepted that this is evidence of a barrier to entry from private school.