> One big problem with the plan, which a lot of people don't realize, is that your typical university math degree isn't that much more challenging than doing well in high school math.

I think this is very US-specific. Here in Europe there's definitely a huge difference. You enroll in a pure math-only 5 year degree, and are talking about topology in your third semester already. That might be a specific thing only my analysis prof does, but quite a lot of time is devoted to neatly delineating different levels of structure you're equipping a space with, proving things with as less structure as possible or at least discussing it. You are expected to juggle tricks on a totally different level, which takes a lot more qualitatively different practice than in HS.

When I talk to people who take math classes in the US it's usually not on that level of engagement until the graduate courses. Maybe it depends on their university, in smaller European countries you often have just like one main math program that is then high-intensity.