Well if there is 200 K people in waterdeep, and 1.8 more million in the area, it seems pretty settled to me.
Also, I took your geographic instruction and made a rough rectangle of land using a map that calculates distances in FR. I was able to draw a rough rectangle and calculate that the area you depict is roughly 55 000 square miles. based on our previous discussion, where 1 square mile can support 500 people with some magical assistance, that's enough land to support 27 million!
Now, of course the land is not that settled, lots of mountains, moors and such, so as a result we don't have 27 million people living there.