I might have misunderstood what you were suggesting; from rereading, it sounds like you're suggesting to rely on something like the end of a TCP packet rather than having an explicit length. If this is what you mean, my concern would be that requiring a protocol to map 1:1 to the transmission protocol's packets (or a similar construct) can be limiting; it would require all messages to fit in a single buffer, which could prevent it from working with clients or servers configured to use a different length and might make it difficult to use with other transmission protocols.

My question in the beginning of this thread was intended to be specifically about general purpose formats like protobuf; I think relying on the semantics of TCP or something like that might be a good choose for a bespoke protocol, but it doesn't seem like a great idea for something expected to be used in a wide variety of cases.