> Everything is just inline, except when messages can be recursive (which should be rare for most protocols).

Many messages are have lots of optional sub-message fields, and set only a few of them in any given message. These messages would be huge if everything is inline (especially if the same thing happens in those sub-messages).

I agree that inlining all sub-messages works great for dense schemas, but it assumes too much about the schema to be a good design for a general-purpose proto library I think. Also maps and repeated fields can never be inline.