I like the idea of having the freedom to do everything but also having intelligent defaults to nudge the programmers in the right direction. Optional immutability is a great example of this. When you make something the default, you send the message "this is usually the right way to go about things".
When immutability is optional, you can no longer rely on it to reason about the program. Specifically, you have to verify that any two pieces of code you're worried about actually don't interact, because you no longer have a guarantee that they can't. I think a lot of restrictions are like this.
Edit: To quote a great comment on the article,
> Invariants make software more reliable. Weaker languages have stronger invariants. There’s less stuff to test.
