What are the pros/cons of using a custom built language/parser such as this(or others like googles gRPC IDL, etc), versus simply creating a DSL(Domain Specific Language) using an existing language like Kotlin,Scala et?

I feel that leveraging an existing language with good DSL support, along with an existing IDE (e.g. IntelliJ) would be a lot more pragmatic, easier to support and to potentially maintain.