Why was it decided that if big_typed_query() is not supported by an sql driver, that we want to throw an error instead of transparently falling back to big_query()?

I normally would say, the transition to big_typed_query() is, by definition, a fuzzy one, because depending on the database (driver), one cannot possibly have a one-to-one typed equivalence for all available database types. So it is inevitable that some values will be typed, and others will still be strings. And with that in mind, you might as well fall back to big_query() completely if there are no typed results.

If there are no objections, I'll alter the code accordingly (at least in 8.1).