Or just keep using protobuf2, 'cause it's been working great for us for ~6 years.

> But you don't have to do either. It seems like you aren't familiar with the use of protocol buffers.

I've written my own protobuf compiler. I'm familiar.

> You just define optional fields with a reasonable default, and magically all the old protobufs get that default value.

That only works up until there's no "reasonable default".