Isn't it funny how we take a relatively arbitrary distance and apply a specific standard?
The marathon is 26.2 miles/42.2 km. Yet we are intrigued by a sub 2 hour. Yes, it is cool, but what if the marathon were 26.1 or 25.75 miles?
If that isn't enough, we reference a sub 60 "half" marathon. It is a HALF of something. It is an even more random distance.
Anyway, I don't think there is anything magical about the 2 hour marathon. It will happen in time. Though, if we use McMillan, we haven't seen a 12:19 5k or 25:34 10k (nor a drop in those records), so I think 2 hours is safe for awhile.