And... most of the important projects are still using proto2 and there is no realistic way to do interop between those two formats. IIRC, you cannot use proto2 enum in proto3 for some obscure technical reason! This creates a backsliding issue; you cannot migrate old protos with thousands of different messages, fields and enums where new proto2 messages to use them are added everyday.

This is an important distinction from the proto1->proto2 migration, which was in a much better compatibility situation yet still took years to complete. AFAIK, this is the main reason why the proto team decided to create a superset approach (edition) so migration can be handled as a flag at each message level.