Author of protobluff [1] here - a zero-copy, mostly stack-based implementation for Protocol Buffers in C. Yes, you must scan through the message to find the corresponding tag/value pair you're interested in, but in many cases, it can be fast enough if you're only interested in a few values of a large message. Sure, using a vtable is much faster, as it provides O(1) lookup semantics, but sometimes you may not be able to change to another format without touching the entire stack.

[1]: https://github.com/squidfunk/protobluff