If the testing is insufficient, as in practice it often surely is, then static typing would seem to be more valuable.

If testing is enough to ensure the software is reliable, does the extra benefit of static typing make it worth the cost? This could be quite a lot of testing! The more testing that is done, the fewer type bugs remain that static typing would have found.

The argument you want to make against this is that static typing would have benefit beyond just finding bugs. The argument you don't want to make is that static typing reduces the need for testing.