I do not agree. Which parts are necessary depends on the application; there is not one good way to do for everyone (and Protobuf is too limited). You will need to implement the parts specific to your schema/application on each end, and if the format does not have the data types that you want then you must add them in a more messy way (especially when using JSON).
In what ASN1 application is protobuf spec too limited? I've used protobuf for tons of different things, it's always felt right. Though I understand certain encodings of ASN1 can have better performance for specific things.