You are right, scaffolding seems like a better descriptor.
> It sounds like you're pointing to something more nefarious.
Well the structure itself is quite nefarious in a way. You have to constantly fight against it to progress and don't really have a choice at the beginning, which often leads to learned helplessness and PTSD in the dropouts. As a teacher you also have to constantly fight against this because any shortfall of effort on your part leaves your students behind in one of those pitfalls, and its largely dependent on the students ability to overcome the torture. You generally aren't given sufficient resources to do this because there's no way out; only through. This is why the structure is nefarious and at the root of the problem.
The unlearning process after learning to competence is imperfect and induces what amounts to self-torture sessions. The imposition of psychological stress (torture) actually lowers the ability for rational thought, and may permanently warp people at vulnerable stages of their lives. Children tend to have a period where they try on various personas after which their identity crystallizes which they carry forward. Adopting learned helplessness at this point makes them a resource drain on everyone. You see these effects in the youth today where they can't even read in many cases.
The sequences in math for example rely on a undisclosed change in grading criteria resulting from this path, a gimmick if you will. There is the sequence, Algebra->Geometry->Trigonometry. Algebra is graded based on correct process, whereas Trig is graded based on correct process and correct answer. When the process differs between classes because the process taught was a flawed version, and you pass Geometry, you can't go back. Its outside the scope of the Trig teacher to reteach two classes prior, and they'll just say: "If you are having trouble with this material you should choose a career that doesn't require this", and leave it up to them. This was actually pushed for adoption by the NEA in the 90s, where they were going to strike if the administration didn't cave.
There are similar structures used in weed-out classes in college as well. Physics used to use a non-standard significant figure calculation when the questions were related by a property of causality (1st answer is used for the 2nd, and the 2nd for 3rd, 2 tests, you can only get 1 question wrong on one test to pass. It must be either of the last two on either test). Using a correct method to reduce propagation of error would cause you to fail, and the right answer was passed around to only the professor's favorites, hence very similar to gnosticism where the only the experts determine who may receive the secret knowledge.
An excellent teacher that constantly bucks the norm will naturally sidestep many of the pitfalls, but an average teacher who is overburdened from lack of resources, and ground down who has sunk to the lowest common denominator of work production won't provide a bridge over the pitfall and these things happen through simple lack of action as a consequence of the adopted structure.
When people speak of nefarious and maliciousness there's often an assumed intent, and in a way negligence can be intent but while some could argue these type of plans conform to this based on things our nation's enemies have said, its probably equally if not more a result of degradation and corruption from within as a result of the flaws inherent in centralized systems.
The history about how this came about is particularly muddied. To give some context, Sputnik in the 1960s shocked the US, and they wrote a blank check for Academia towards more engineers and math alumni. It was a problem you can't fix though using money, and when that was noticed the hiring standards which were quite high in the 1960s, were lowered. Whether the lower standards caused this, or subversives snuck in as an attack on the next generation, no one will know. The effect though is by 1978 there is a marked difference in the academic material published prior and after with lower quality resources being available after which conform to the mentioned flawed pedagogy.
The proposed alternative is to go back to the classical pedagogical approach. Use real systems, teach the process of reducing those systems to first principles (in guided fashion), creating models, and then predicting the future behavior of those systems, identifying the limitations. Some professors still do this, but they are in such a minority that you may only see on or two in a local geography (driving range/county) across all areas of study.
> Sounds compelling but it strikes me more as a limitation of demand for good math teachers.
I've known quite a lot of extremely intelligent people who have been hobbled because they couldn't get through the education, the few that have are often unable to apply the knowledge outside a very limited scope. Its a bit of a chicken egg problem, you need the chicken first.
The hiring standards were never raised back up and remain low, and the materials used to teach those have degraded, there is also no incentive towards improvement of teachers. Basic performance metrics are eschewed from collection. You see this particularly in colleges where they may collect pass rates but won't differentiate a person who has taken the class in the past from a new student.
There are also other incentives which are covered quite plainly in the documentary "Waiting for Superman" in the Lemon walk. If you don't fire your lowest performers, and they are effectively guaranteed wages without the appropriate level of work, they end up driving the higher performers out through social coercion, harassment, and corruption. The higher performers make the lower performers look bad.