Barrie England's answer is very good, and I don't think I can add much to it. As he said, it is idiomatic to use accusative-case pronouns for any kind of predicative complement, as in "You are me" or "He was me." You can hear sentences like this all the time from native speakers (well, not exactly like these; these particular sentences are pretty weird in terms of what they mean). This is mentioned in good grammar references.
I don't know about the tendencies in usage. The modern predicate-accusative construction is not particularly recent; it's at least more than a century old. The predicative nominative doesn't come naturally to native speakers, but it's also not completely dead nowadays, and while it's difficult to predict the future, I don't think the predicate nominative will go entirely extinct any time soon.
The main reason I wanted to make this post was to correct a misapprehension in the original question. "It is me whom you saw" is somewhat awkwardly put together. Prescriptively, a personal pronoun and a following relative pronoun are supposed to be assigned case separately, the personal pronoun based on its role in the matrix clause and the relative pronoun based on its role in the embedded clause. Examples: "Let him who is without sin cast the first stone", "I, whom you scorned, have now triumphed." There is an observable tendency for people to alter the case of the personal pronoun to match the case of the relative pronoun, but this is not generally considered to be correct standard grammar.
Since people generally don't use "whom" unless they're aiming for prescriptively correct grammar, "It is I whom you saw" would be better, in my opinion. Colloquially, "It's me you saw" or "It's me who you saw" both work.