I think you're on the same page. The gist is that while you'd hope to have a stable interface, in reality things change. By marking fields required, they basic

I think you're on the same page. The gist is that while you'd hope to have a stable interface, in reality things change. By marking fields required, they basically have to stay that way forever so you can support older clients. In doing so, you create a brittle API that may not be able to evolve with changing requirements. For this reason, I believe Google got rid of required fields in proto3 (everything's either implicitly optional, or a repeated field which may have 0 elements).