Maybe not in vanilla text editors, but I don't see a reason you couldn't have an Emacs mode or special-purpose editor that lets you manipulate protobufs as text or structured data, respectively.

Actually, there is a built-in ASCII format for them as well. It's a little less flexible than the binary serialisation though.

There's a canonical JSON serialisation in v3 as well, although obviously that doesn't solve the OP's problem...

Proto3 has a JSON serialization that you can use as an intermediate for editing