> "Oh, we're slowly building up a custom, buggy, binary framing protocol. Lets just use protobuf/msgpack"

By using protobuf/msgpack you lose the ability to precisely control the layout and encoding of your data on the wire. Most applications don't care, but this results in your wire representation being defined by "whatever protobuf says".

Say I want to transmit an unsigned 16 bit integer with protobuf. How do I do that? The documentation doesn't include 16 bit integers as a datatype, so I'd probably have to wrap it in 32 bits and/or use some varint stuff. It would be simpler to just write a big endian 16 bit int though.

I wish there was a simpler alternative to protobuf that gives more control to end users and doesn't try to be smart. Until then, making your own binary protocol is not over-engineering.