Issues with predicate nominative

As far as my understanding goes, English does have a predicate nominative for the copula to be as well as semantically related words (to become, to seem) if the entity in question plays the role of subject in the activity it is involved in.

It is I who stole the hen's eggs.

It does use the oblique case if the entity has the role of object

It is me whom you saw.

However, the following defeats that rule

You are me

There are a bunch of songs with that title. Same for

He was me.

• Would it be correct and idiomatic to say "You are I? /"He was I?"
• Is there any mention of that behavior in grammar references?
• Is the predicate nominative on the decline in general (have there been tendencies over the centuries)?

The circumstances in which anyone would say either ‘You are I’ or ‘He was I’ are hard to imagine. Conditional clauses offer a better test, where we are, I would guess, more likely to find if you were me than if you were I and if he was me than if he was I.

In a section headed ‘Variation in pronoun choice after forms of be’, the ‘Longman Student Grammar of Spoken and Written English’ speaks of

. . . a tendency for the accusative form to spread in popular usage into contexts traditionally reserved for the nominative form.

and confirms that

. . . the accusative form is the normal choice in practice, in both conversation and the written registers.