Looking at the protocol definition, it looks like completions aren't capable of performing anything more complex than a simple text insertion or replacement.

I don't think that's going to be sufficient for a lot of reasonable use-cases.

For example consider the completion of the name of a type in a Java program. When inserting the name of the type at the place where the cursor is and where we invoke the completion... we probably also would like the import statements at the beginning of the file to be modified to import that type if necessary.

Completions like that require support for edits that can modify multiple non-contiguous regions of text in the document.