JK Rowling pens a Harry Potter prequel / War of Roses/Holmes?Figg/Walpurga

[Carol]

<rest of post snipped>
> Miles, who still doesn't know about "it's me!" or "it's I!", though
the latter sounds wrong

Carol responds:
"It's I" (or "it is I," if you're going to be really formal) is
technically correct but no one says it. Fowler's ("The New Fowler's
Modern English Usage," third ed.) lists it as one of several instances
"in which *me* is now tending to usurp the territory that logically
belongs to the subject-pronoun *I.* . . . . *It's me* in an answer to
the question *Who is it?* is now standard [and] in answer to the
question *Who's there?" the natural answer is *Me* not *I.*"

However, Fowler's lists a number of other instances of *me* as
usurping the function of *I,* some of which are clearly ungrammatical,
such as "me and the teacher" or "me and the lads" used informally as
the subject of a sentence, so the section is clearly descriptive
rather than prescriptive. However, rest assured that if you say, "It's
me!" you'll sound casual and natural and no one will judge you for it.
If you say, "It's I!" you might well be classed as a grammar snob.

BTW, if you're ever unsure whether "who" or "whom" is correct, always
choose "who." "Who" used incorrectly could be passed off as mere
informality; "Whom" used incorrectly, as in, "Whom is speaking,
please?" is a solecism.

Carol, now wondering whether the subjective (or nominative) case is
going the way of the ablative, dative, and accusative cases, all of
which have already been "usurped" by the objective case