I studied math & computer science at university (15 years ago) that was quite selective in CS admissions (probably filtering out ~80% of high-school graduates).
At least half of these already self-selected and 80% filtered students were not really able to program (and eventually flunked or transferred to other departments - although some people who could program flunked and some who couldn't graduated, I think the two groups were quite well correlated).
I'm sure you can do better at teaching (and filtering) than the CS program at my university, but if such a focused group had a 50% failure rate I'm sure the general population is at the very least not better (remember that it's not just about 'raw' ability but also about interest and motivation).
At least half of these already self-selected and 80% filtered students were not really able to program (and eventually flunked or transferred to other departments - although some people who could program flunked and some who couldn't graduated, I think the two groups were quite well correlated).
I'm sure you can do better at teaching (and filtering) than the CS program at my university, but if such a focused group had a 50% failure rate I'm sure the general population is at the very least not better (remember that it's not just about 'raw' ability but also about interest and motivation).