Why should a copula link two noun phrases of the same case?

Question

https://english.stackexchange.com/a/30392/50720 motivated this question:

To quote from the clear explanation:

The rule for what [Fowler] and others consider technically right is ... that "to be" should link two noun phrases of the same case, whether this be nominative or accusative. ...

Sometimes in English, though, "to be" does seem to have the force of a transitive verb. ... The occurrence of "It's me", etc., is no doubt partly due to this perceived transitive force. ...

The final factor is the traditional use of Latin grammatical concepts to teach English grammar.

Grammar Girl similarly broaches this rule, but does not explain it:

...The traditional grammar rule states when a pronoun follows a linking verb, such as "is," the pronoun should be in the subject case. It’s also called the “nominative.” ...

So I'd like to ask about the genesis or rationale for this rule? How can I rationalise it?

Answer 1

"Is" equates. Equals should be similar. What's on the left side of "is" is the subject. What's on the right side is therefore subjective case, not objective. Is that a simple enough rationale for you? Somebody else can give you historical background on how and when this perfectly logical rule came into being.

Answer 2

Objective forms are used colloquially - rightly or wrongly - when the word is not the grammatical subject of an expressed verb, even when the subjective form is logically correct: so we hear "Me make the dinner? Ridiculous!" as well as "That's her!" and "'Which boy stole your bag?' 'Him!'" So 'be' does not have the force of a transitive verb; but complements can - rightly or wrongly - be objective in form even though subjective in meaning. Rogermue may well be right that French idiom is one source: but another is probably the accusative and infinitive construction in reported speech.

Answer 3

One argument might be that "to be" is followed by a nominative in Latin, Italian, French, German, Dutch, why should it be different in English.

But, as I often hear the argument English is no Latin, it would be useful to have one more argument.

• Who was Henry VIII? Possible answer: He was father to Elizabeth I.

We would ask: He was who? and not He was whom?

As to "It's me" I agree with David Garner and take it that it is influence of French C'est moi. So this "me" is no accusative but a special idiomatic form infuenced by French and it should have a special name, e.g. French use of me. The moi in C'est moi is a nominative. Moi derives from Latin mea (persona), my person.

The rule quoted from Grammar girl is, as so often, only half of the matter as it does not say that in spoken language "It's me" is used, probably more frequently than "It's I". And, of course, such curious use of "me" must be explained. The reader should not have the impression that it is a normal accusative.