Oh really? There's no way a medical school can discern a barely marginal student from a high performer without GPA?
Of course they can but that's irrelevant. People who may put a sharp knife in your body are expected to be among the best of the best. Grades for a basis to discern "pretty good" from excellent, and the latter still aren't guaranteed spots in top programs. I think the guy who coasted through with the bare minimum can be culled, but that means a huge increase of effort just to determine who the bottom feeders are. Harsh I know, but that's the reality. As these schools determine who to accept those with "almost the right stuff" aren't on the list and pass fail makes selection far far harder.
What will happen is recommendations will count for even more and if you do very well but your prof doesn't like you? Who's to say you weren't marginal? It's not like you will ever know what's in the recommendation.
Are grades the only thing? No. That would be one major element, but top tier schools will look at the undergrad schools reputation and their past track record as far as how the recommendations lined up with the reality of the person put forward as a candidate. They'll look at your personal life in your activities such as volunteer work if appropriate and the interview for the final cut.
Some schools do have metrics other than grades which are used where appropriate. My son is a beneficiary of that kind of thinking. His grades were good, but he's not a bookworm striving for the 4.0. Instead he plays LOL more than I would have liked but that's how he is and he's far from being in trouble academically. His college has a very selective honors science program and they do not select based on the highest high school GPA. They do have a lower end cutoff but it's not draconian and BTW this is done before admission because some people apply to get in this program and the college gives notice of the results well before most other college's deadline.
The process takes an entire day and students are placed in groups and tested and observed. Not a paper test, but "here is a situation, figure it out". During the morning individuals are given several scenarios and the students tackle them as seems appropriate. After lunch the individuals return to the group and discuss what they did and their thoughts. Then there was a group project where they figured out who would take what part of the exercise and everyone would go back and discuss further until time ran out or they reached an answer.
The problems were somewhat abstract so there wasn't a clear right or wrong. When that was done each applicant was called in an a panel of professors, one from physics, chemistry and biology looked at the results and questioned as to why they did this or that.
Done.
Three weeks later students got calls to let them know if they were one of the twenty or so. Obviously my son got in. Yes this is me being proud however I believe relevant. This 10 hour process wasn't about solving a problem it was about thinking. Who had insight? Who could do well with the abstract under pressure. Who thought best outside the box of normal constraints and conditions as seen in all of their prior education. Who was a leader in the group dynamic and was it positive or negative? Could these people work with one another and on and on. The school wanted the best minds not the best grades for this program.
Looping back to the start- hey here's no grades being given. But pressure to perform? It was there in spades. There was no "feel good" and it was very much pass/fail. The kids who were tested loved every minute of it because they had a chance to put their brains to work. It was fun.
Now multiply this rigorous pass fail scenario by many orders of magnitude and then one might have a basis for pass/fail in the sciences, but with other areas? I'm not so sure and the amount of effort put into something like this is staggering. No one has the time and resources to scale this up for an entire college or university.
Bottom line, grades are a filter, an imperfect one perhaps but useful for going beyond undergrad especially for top tier programs.
Not everyone is going to feel good.