This is a great question. I think the best answer may not resonate with DC until after they have experience of university.
With rare exceptions (eg a bereavement or breakup during exam season) better exam results, particularly in core subjects, will prime you for a better university experience. My biggest worry about those admitted with substandard grades concerns their university experience.
I came up under a system where a state exam carried great weight. Top scorers got preferential university admission. The exam was largely a test of intellectual power, and I arrogantly but accurately assumed I would ace it. Therefore I slacked off badly in high school. I took care not to disqualify myself from the top uni tier (assuming top exam performance) but nothing more.
Well, I chose Nerd Tech U and all my classmates seemed to have been studying for 60 hrs/wk all through high school. I struggled until I lucked into a course module thought hellish that totally clicked with my thinking. The lecturer was an influential professor and he both gave me confidence and paved my way.
This turned my life around, although if am honest the professor’s glam, young wife in a related field - we had few women in STEM in my country and almost none were physical role models - was also a factor.
But I digress. The point is that meeting the admissions standards means that you are prepared to meet the challenge, at least in theory. And if not, not. The teaching pedagogy is, or should be, designed for the band of students who meet the admissions standards.
Everyone tries to justify the need for bums on seats as giving others a chance. That’s true. I know that when it’s a question of work habits only a (gratifying) minority adjust. I would think that amongst hard workers even fewer do. Ultimately money is driving these. decisions