I am wondering what the typical wastage rate is when a subject is taught but not taken up as a career.
I don't think it's relevant. Education is never "wasted", even if it's not used directly (even in a non-career role). For most kids, everything above elementary mathematics and a grounding in English is totally irrelevant at a career level, but that doesn't mean we should filter those kids out early (even if we could) and push them into totally vocational education from the start of secondary school. It's the educational equivalent of "You'll never do any better than this, here's a spade, dig me a hole", and ICT is a reflection of exactly that attitude. Don't teach the "why and how" of computing, teach the intricate mechanics of how to format a spreadsheet in this particular package. It stinks.
History, geography, languages, art, technology, music, higher math, science, literature, computer science, these are all subjects that help to make rounded individuals, healthy members of society. Education shouldn't be about shitting out an endless stream of nice compliant little worker bees, it should be about helping children to achieve their potential and make the transition from child to adult.
 Yes, I'm aware of the difference between "wastage" and "waste", but the use of the former implies to some extent that the time spent teaching (for example) science would be more usefully spent teaching (again, for example) better pickaxe and shovel technique.