If you have bright enough undergrads, you change the curriculum for them within their field of expertise, so that they still get the breadth of things outside it while not wasting time with things they know. You let them not take as many classes, take graduate courses, do more research, take more courses from other departments in related areas but with different perspectives, and so on.
When I was an undergrad, in physics, there was a professor in the department who had done his undergrad there and was legendary, as was quietly mentioned in awe, for not taking any undergraduate physics courses while there; the department had let him skip all of them, and instead take graduate courses and do research.
Undergrads who do research generally aren't very good at research yet. A major reason is they either lack or don't fully understand the pre-reqs, which they progressively and cumulatively learn during undergrad. A student can be incredibly smart, but acquiring a strong rigorous math background will still take years.
I can see a committed and gifted student being able to get most of the pre-reqs for doctoral studies in America or Canada while in high school.
Anyone can try doing research, even undergrads who half-know the foundations. However, trying research doesn't mean you have the background to do great research or to succeed in a postgrad program.
Plenty of people in postgrad programs don't know the foundations. It's ok. You are there to learn.
Completely unfair to expect someone already doing research to slog out 4yrs of classes not furthering their career.
Even at top universities, very very few freshmen are capable of doing high-quality research immediately. They'd be better served learning the foundations inside and out with a cohort of similarly strong students to challenge them.