According to the blog post of one of the guys who worked on proto3: the complexity around versioning and required fields was exacerbated because Google also has

According to the blog post of one of the guys who worked on proto3: the complexity around versioning and required fields was exacerbated because Google also has “middle boxes” that will read the protos and forward them on. Having a contract change between 2 services is fine, required fields are probably fine, have random middle boxes really makes everything worse for no discernible benefit.