First of all, I agree that the accidental argument is not very convincing (two developers writing a method Launch(), one that launches a football and another that launches nuclear missiles).

However, we lose two important things with implicit interfaces:

- Code readability. It becomes much harder for a human to interpret what types are actually implemented.

- More importantly, it severely limits the tooling available since the compiler has a lot less knowledge about the types you are dealing with. Automatic refactorings are all but impossible with structural typing.

Overall, I really don't see the harm in saying explicitly "My type is called Account and it implements Serializable and Entity" as opposed to me having to guess by reading all its methods and also having to remember which methods are necessary to be an Entity or a Serializable.