I think looking up values based on a simple offset is far simpler to all the clever rules and "parsing" that happens in capn' proto (some of which no doubt caused by the desire to support streaming "automatically", an anti-feature IMO, streaming should happen at a higher level to take advantage of domain knowledge).

I really don't see how you can even pretend that cap'n proto is less complicated. Yes there's an extra indirection at runtime, but the benefit is great simplicity, and quite possibly better performance when the vtable is hot in the cache.