There is a definite difference between tests/courses/other hoops to jump through that are required in order to do a particular job and the ability to do that job successfully. I am not sure that any of the standardized tests we encounter in life actually test something other than test taking skill + some base knowledge that can probably be looked up if needed outside the test setting. I mean, we definitely want doctors who can diagnose without looking every other symptom up and lawyers who can answer questions in less than an hour, and engineers who can give a back of the envelope answer without extended consultations, so base knowledge is important. Since it is easier to test in a multiple choice format for large cohorts, we prioritize this over other forms of testing for admissions to various programs.
Same with classes--doctors probably don't need to know so much organic chemistry, engineers might not all need a deep understanding of algorithms, and physicists can probably succeed without two semesters of biology. In all majors, students probably don't NEED as much depth in every sub-field as is required for a typical degree. BUT, the ability to at least pass a class in these things is required for graduation/admission to the next program. So therefore, someone who wants to be a doctor, for example, needs to pass organic chemistry and do well on the MCAT whether or not these things are indicators of later success as a doctor in and of themselves.
This is true throughout the training and credentialing process. To get to my lofty position here at ProdigalU, I definitely had to take classes I didn't not enjoy to learn material I have never used since I completed the final exam. I had to take exams that have no real relationship to the things I did in grad school, let alone as a professor. It doesn't matter in the end. These things are required as hoops to jump through along my career path, so I did them. I actually do believe that these sorts of hoops have a kind of predictive value--they predict whether someone can buckle down and do them to a minimum level of competency. I would bet that doing what needs to be done competently is predictive of future success.
While I am sure that at least some of the students who do poorly in my class would probably be good doctors/engineers/scientists (the sort of people who are required take my classes), the truth is that most students who work hard can earn at least a B+ if not an A. Those that can't despite their hard work are unlikely to be able to jump all the hoops required between my class and their goal, and they may as well find this out sooner rather than later. Similarly, those that can't muster up the will to do the required work for my class may also have issues mustering up the required will for the unpleasant or difficult parts of their future career. Sure, people change over time, and that is why I know people who went to various professional schools later in life after not working hard in University, but the key word there is later. As in after they learned how to work hard when required, and how to do so effectively.
Most of the students who come to my office to complain about how my class ruined their life are students who never used the tools provided to help them succeed in class. Sometimes, I have never seen them before, even after a whole semester of lectures! While I definitely have sympathy for the (relatively rare) students who work very hard with little to show for it, I also am pretty sure that someone unable to master the material for my sophomore level problem solving-based course is unlikely to do well in higher level courses that build on this material (and by little to show for it, I mean C or lower despite loads of hard work). I've definitely had students go from F the first time to A the second, but that is fairly unusual. Most of those students either didn't work the first time, or worked ineffectively and learned better study skills the second time around. One student told me that failing my class was the best thing that happened academically in the end, since the shock of it motivated the student to actually listen to advice on how to study effectively.
I don't think encouraging students who work very hard and follow advice (from me and/or study skills resources at ProdigalU) on effective studying to keep on the path they are on is a kindness to them. I don't offer students unsolicited advice unless I am in a supervisory role beyond course instructor, but if asked, I will say so. Hoops may not necessarily be predictive (or even fair), but they are required. It does no favors to pretend otherwise.