I disagree. I was able to get into one of the best graduate programs in my field a few years after a mediocre undergraduate record. This took some preparation, but it came down to finding other avenues to prove myself. The committee knows your undergrad record doesn't mean much if it was many years ago. "What else have you got?" they might ask. It takes some effort and creativity, but it can be done.

Adults with a newfound motivation are valuable and they know that. But just saying so isn't enough.

That's awesome that it worked out so well for you! It's not something that I've tried myself, just the impression that I have. I do wonder though if grad school admissions might be another story compared to undergrad, since they probably see a lot more older applicants with work experience and are possibly more used to adjusting their evaluations based on what the most recent/relevant signals are (just as businesses do when hiring).

But leaving during undergrad (or just never going to college) then excelling in self-learning and industry seems to put you in a much more awkward position when it comes to continuing onward with education. You are far beyond most undergrad-level courses, but in most cases you need that bachelor's degree to even be considered for grad-level programs--or at least I believe that's the case? You end up needing to waste years of your life and tons of money just to qualify for the courses that would actually teach you something new.

So perhaps the problem is as much that there just aren't compelling tracks offered to people who don't fit neatly into the lines as it is about admissions flexibility.