For something without high performance requirements (like the command&control messages for this debugging adapter) I don't care too much about the efficiency of

For something without high performance requirements (like the command&control messages for this debugging adapter) I don't care too much about the efficiency of the serialization and the protocol. But what I would care about is that it's easy to create a protocol specification and to generate most of the boilerplate code out of it - so that the schema gets automatically verified and that developers can focus on building their application logic instead of getting the serialization right. That can be done with JSON too (e.g. with json-schema or by generating code out of typescript definitions). However I haven't yet checked whether LSP and the debug protocol make use of this.