In other words, the parents should get a return on their investment?
Your child is not entitled to an Oxbridge place over a state-educated child because they might have more potential and ability, they're entitled because you paid extra for it?
Anyway. If Oxford is going to pass on those kids, who are often multiple years ahead of the average, some other university will accept them. And then, this university will likely beat Oxford in ratings.
If, however, you want to convince yourself that the amount of money you've spent on your child's education means they're smarter than the rest, go right ahead and believe that.
Universities don't select for whether a candidate has "reached their full potential". They select for what that potential is.
For smarter. My narrative is more prepared, used to working harder. And also, it's self-selection. If you are not made for that type of education, you are going to leave.
No matter if universities select for potential, or operating near it. Both would be nice. Now, it is increasingly also how they will shape the society if they accept student A instead of student B. And they want to shape society in a way that discriminates against certain kids for who their parents are.