If you look at a queue at any random time, it will have necessarily increased more than it has decreased over its lifetime, otherwise it would hold negative items.

That’s why you look at the average age of the items in the queue.

That's a good measure (average latency) but that's very different from your earlier statement of "heading in the direction of empty".

Well not really. If the average age is increasing it’s not heading in the direction of empty.

You still haven't defined what "heading in the direction of empty" means. Like I said, your queue can't be decreasing in size over its history, and it cannot be decreasing in size most of the time either, if it started empty.