Anecdotal, but it seems universities have solved this by figuring out which schools have grade inflation. I went to a gifted school in Chicago that was quite competitive and did not have grade inflation. 20% of the school went to Northwestern every year because they'd accept every B student.
For one thing, if a school has grade inflation so bad that even an A+ from that school isn't enough to get into Yale - is that a problem?
For another example, if adjustment for grade inflation means Yale will ask for an A+ from Martin Luther King High, Detroit while they'll accept a B from Phillips Academy, Andover - is that a problem?
Well the thing is I went to an inner city public high school. It was much closer to "Martin Luther King High, Detroit" than it was to a prep school. Majority of students were below the poverty line, yet almost half were accepted to Northwestern every year, many with full rides.
I've met people from elite private day schools. Their education in a different world than 99.99% of public high schools, except maybe a few like Stuyvesant in NYC and Lowell in SF, or the fortunate few where 3/4 of the kids have parents who are doctors or college professors (why not both?).
