There is you

I was listening to Billie Holiday on the radio yesterday and she sang this lyric "as long as there is you, you and I" and something occurred to me.

People always say you must use the number of the verb that matches what they call "the real subject" of existential statements. "There is a dog in the garden, there are dogs in the garden". "There are problems in Syria". Never "There is dogs, problems, people..." (although if with "there's" you often hear plurals - "there's problems", for instance). But I don't believe I've ever heard anyone say (or sing) "Till there were you" or "As long as there are you", though of course "there you are" is the only way to say that.

And "there am I" - in existential constructions - is also none existent.

"Who is there to stop them?" can't be answered with "there am I, ma'am" or "there are you, after all". It's "there is" all the way.

So now the question is, how long ago did that happen? Chaucer answers the question "Who is it?" with "It am I" - not just "It is I" but "am". Most people today say "It's me". Is the existential, already deeply conflicted, on its way to treating "there" as the subject? Will "There is problems in the world" one day be Standard?