The author doesn't really respond to the point, he simply says they were non-trivial in complexity, which may be true, but that doesn't mean they're non-trivial in their dynamacism (is that a word?). Moreover it in no way responds to the claim that these codebases aren't very good.

Does it matter if "these codebases aren't very good"? Have you ever actually seen a good code base? Most code I've seen has some poorly written corners; something where someone was in a hurry, or didn't know what they were doing, or someone inexperienced with the project started working on it, or the like.

The point is, these were real-world codebases with substantial unit tests, and they had type errors that weren't caught by these unit tests.

Honestly, in several groups I've worked with, I've had trouble getting people to add unit tests even to code in dynamic languages. A codebase which already has substantial unit tests is likely to be better than average, on that basis alone.

This is about saying, in the messy real world, do unit tests actually make type safety obsolete. And the answer is, no, they don't, even code with fairly good test coverage can be improved by adding typechecking. Now, there is the question of whether writing in the dynamic language allowed people to write code faster (it's generally a lot easier to translate correct code once it's written than to write it in the first place); or whether some of the more highly dynamic features of dynamic languages benefit writing or deploying code.

Dynamicity or dynamicness.

What's wrong with "dynamism"?

I considered it, but dynamism refers to personality and philosophy, while dynamicity is just the condition of being dynamic.

The Wikipedia disambiguation for Dynamism (last edited in March) includes:

"Dynamism (computing), When any process in computer is using Dynamic management methods for its processing/computing/memory management/parallelism handling for being able to give more user friendly work that are more easy to interact and modify."

... for whatever that's worth.

Consider:

  staticity
  staticness
  statism

That doesn't work because statism comes from "state", not "static" - I don't think there's a comparable derivation for "dynamism".

If most code doesn't use "dynamacism" (we'll pretend it is a word), then it isn't an important value for most people. You are suggesting people should use dynamic languages to gain no benefit because some other software might in theory benefit from it.