Firstly, I do not see in the underlying research that a B from indie equals a C from state, so please can you point out this specific? Because if not then you start comparing apples with oranges.
I don't have a doubt that - ceribus paribus - a B from comprehensive trumps a B from Indie. But that doesn't therefore mean that a C from a comprehensive = a B from private, and without that clear statistic, you don't know if the C state student is capable of the same achievement as the B private.
Interestingly, when you look at the underlying research (which compares outcomes from the same input grades) there is a finding that when you look at 2.2 and above, the statistics are reversed and 67% of private achieve this against 62% of state, which would suggest you ought to do the reverse of your suggestion, OP, and let in more indie students in order to shift the bell curve of acceptable degrees higher.
The reality of course is that whilst there are so many variables, universities should try to take into account those variables as much as they can and the research shows that they do - indie pupils are required to achieve slightly higher grades to enter uni than state, according to the actual underlying research (as opposed to the headlines) ABB on average, vs BBC.
What is a real shame is the the research showed far more starkness of differences between the races, which warrants real investigation and action, but no-one clearly gives a shit about that when they can jump on the class bandwagon.