>When you see a huge codebase in python grown over several years and try to refactor it, reason about it, or just enhance it, you will inevitably come to the conclusion that the omission of static type checking is a grave error from a system-design perspective.

I inevitably came the exact opposite conclusion. Perhaps because at the start of my career I was trying to do the same thing with Java, and it added overhead and expense without a commensurate payoff in safety.

Frankly, I think velocity is ridiculously underrated:

* Because of the age old "customer doesn't know what they want" issue - hence you throw away a lot of the code that was not fit for purpose.

* Also because the right kind of architecture is only obvious in retrospect - hence you throw away a lot of the code that was not fit for purpose.

* And finally, because the only times in my career where I've had significant success in refactoring a large mission critical codebase it was python and it was because I'd managed to quickly build custom tooling infrastructure (e.g. testing) to support that refactoring.

I do take some issue with python's type system (e.g. the whole truthiness thing), but on the whole it balances safety and velocity in a highly pragmatic manner.