the percent of such individuals holding engineering as opposed to management, financial or other positions, and using more than Excel and eighth grade level mathematics (arithmetic, a little bit of algebra, a little bit of statistics, and a little bit of programming) is less than 25% and possibly less than 10%.

I would state this differently. Borrowing from the Pareto principle, one could conjecture that 80% of mathematically advanced work in the economy is performed by less than 20% of STEM graduates. The remaining 80% of STEM graduates do not get the economic opportunity to apply the skills which they trained for and end up doing less prominent work (e.g. middle management).

As the OP and others have pointed out, there is a lot of anecdotal evidence to support this conjecture.

But it is hardly surprising, and it is not limited to mathematical talent.

Take management, for example. Just because you studied business in school, does not mean that you will be an executive. I would guess that less than 20% of MBA graduates manage 80% of economic resources (senior executives, bankers, consultants, traders, etc) , while the remaining 80% of MBA graduates are left managing relatively small and inconsequential activities.

Similarly, I would bet that less than 20% of design school graduates do 80% of the design work in the economy. I bet that less than 20% of classical musicians perform 80% of orchestral music. Less than 20% of programmers implement 80% of software used. Less than 20% of athletes win 80% of medals. Less than 20% of science graduates produce 80% of scientific research. And so on.

OP's conclusion is that, in light of this dismal reality, students should not bother learning mathematics after the 8th-grade level (except for "those who need it"). Well, if we apply the same logic across all disciplines, then the OP should conclude that all forms of education should stop after the 8th-grade level for the vast majority of students (and only a minute fraction should need to pursue higher education). That is exactly what the state of education looks like in undeveloped feudal economies, and this was also the state of Western education until relatively recently. I don't think I need to expend a lot of effort convincing anyone that this a socially, economically and ethically terrible idea.

I'll also point out that there there are a couple false assumptions implicit in the OP's original, imprecisely worded conjecture. Firstly, advanced industrial mathematics is not the exclusive preserve of traditional engineering. The generalization that "engineering positions" use advanced math and "management/financial positions" use 8th-grade math, is obviously false. Many areas in finance require advanced mathematics (derivatives, trading, fixed income, etc). Much of actuarial science also depends on advanced mathematics. Marketing, management sciences and operations research are also steadily moving towards advanced analytics. Secondly, it is a false assumption that use of Excel implies that the underlying mathematics is limited to an 8th-grade level. For example, in finance, it is easy to find Excel add-ins for performing highly advanced mathematics (e.g. stochastic differential equation solvers for derivatives pricing).