> Expressions like "Chapter~3" should be output using special commands,
 > preferably using a label \chapter{foo-bar}, or whatever, so that they both

this is true, but in my opinion irrelevant for the discussion at hand. what
you are pointing out is how a document should be logically tagged but we are
talking about a different layer of a document.

your opinion is that natural language doesn't contain digits; those who have
argued believe you are mistaken. Just like `a' might be semantically a
variable belonging to a formula, even if the formula is just consisting of
this variable, it might alternatively be the letter a or indeed the word
a. Why do you accept the latter but claim that this can't be true for digits?