This is entirely what I'd expect, if the current rate is the same as the average rate then no adjustment is necessary. Are your proposing that if solution times remain consistent, we should retarget? Should we make the challenge easier or harder in such a case, and by how much, exactly?
The threshold is not connected to the speed of block issue and a chain length. You can build a 5000 block chain with solution time equal to 60sec and time separation between 0 and 5000 block issue times will be less than an hour.
This is not the case in bitcoin. Bitcoin threshold is linked to the block issue time. If you are building a long chain of bitcoin blocks then the threshold will grow up rapidly and complexity of a new block calculation will grow also. But here we see a strange situation when the block solution complexity will not be changed even if blocks will be issued very rapidly. This is what am I talking about.