Why is the accusative case used for a "topic"?

If I were to write a book about myself, Me would be a more natural-sounding title than I. Also, we say the us-vs.-them mentality instead of the we-vs.-they mentality.

There are a couple of ways to see this:

(a) the notion of "I" being 'nominative', "me" being 'accusative' etc is really something of a bastardisation, or at best a huge oversimplification; do you actually have any motivation for this analysis, or did you just 'hear it somewhere'?

(b) even if you were to conclude that "me" is always an 'accusative' form, who says that the accusative can't be used for "titles"/"topics"?

Or put another way: the fact of the matter is that English uses the form "me" both for the pronoun when used in 'isolation' and when it is the complement of a verb/preposition (as well as other uses). So:

Either (a) you analyse it as being accusative, and incorporate in your definition of 'accusative' this particular use.

Or (b) you analyse it as not being accusative, possibly taking this use as part of your reasoning for not doing so.

Something for you to think about: given that English nouns and adjectives don't decline (as they do in other prototypical languages with case systems), what does it really buy you to talk about 'nominative', 'accusative' etc in English?