> Why wouldn't you want to explicitly number fields? Protocols evolve and get extended over time, making the numbering explicit ensures that there's no accidental backwards compat breakage from re-ordering fields.

ASN.1 went through this whole evolution, and ended up developing extensive support for extensibility and "automatic tagging" so you don't have to manually tag. That happened because the tagging was a) annoying, b) led to inconsistent use, c) led to mistakes, d) was almost completely unnecessary in encoding rules that aren't tag-length-value, like PER and OER.

The fact that you are not yet able to imagine that evolution, and that you are not cognizant with ASN.1's history proves the point that one should study what came before before reinventing the wheel [badly].