Dynamic type systems are not inherently more open
I've come to the conclusion that the benefit dynamic typing brings to the table is to allow more technical debt. Now of course technical debt should be repaid at an appropriate moment but that appropriate moment isn't always "as soon as possible". Let me illustrate, say you're adding a new feature and create lots of bugs in the process. Static typing will force you to fix some of these bugs before you can test out the feature. Then while testing out the feature you decide that it was a bad idea after all or that the feature should be implemented completely differently. So you scrap the implementation. In this case fixing those bugs was a waste of time. Dynamic typing allows you to postpone fixing those bugs after you're more certain that the feature and its implementation will stay.