[ericelliott]

First of all, you're wildly over-stating the difference. The scale based on percent of time spent or saved. It's impossible on this scale to even say that TypeScript is three times worse than no types, let alone thirty times.

As for the bug reducing benefit, yes it's up to 20% if you do nothing but TypeScript. As explained in the article, 80% of bugs can't be addressed by static types at all, so you can't skip the other measures safely. The zero point includes the other measures, which easily cover 90%+ reductions, combined.

Since other measures catch type errors, too, we apply the TypeScript reduction after discounting the other errors caught. Even with the maximum benefit of the doubt, assuming TypeScript catches 20% of the remainder, it's still a small percent of a small percent. You get exponentially diminishing returns with each bug prevention measure you apply, and TypeScript can't catch the lion's share of bugs, so it's the only one that makes sense to leave out as a quality control measure.