Guess what happens if you have a MongoDB ObjectId (as string)

But that's a matter of semantics, not static typing. You're going to have to do type conversion on the string regardless, because "MongoDB Objectid" isn't a native type for the compiler. But with static typing, you need to tell the compiler the type of the variable in addition to telling it how to do the type conversion: for example, in C++, you might have to say...

    var MongoObjectid id = new MongoObjectid("somestring");

..whereas in Python, for example, you could just say...

    id = MongoObjectid("somestring")