Looking at it from a development standpoint, I believe there is more room for improvement in the 800.
If prize money in single track events catch up with the marathon, 1:39.xx will happen FOR SURE within a matter of 5-10 years. You have to admit that the recruiting, development, and competition for the marathon is off the charts right now, while events like the 800 is like the forgotten toy buried in the closet.
Take a look at the level of selectiveness and specialization there is currently in the marathon. It has developed to such a degree that only a small pocket of the world can produce the best in the world despite the valiant efforts from almost everywhere.
The 800, on the other hand, is much more open. There are so many different body types represented in a WC final than the top 8 marathoners in the world. This tells me that the place in the world with the truly optimized body type for the 800, has not fully applied itself yet. This is not to discredit Rudisha type people, its just that there are too few Rudisha type people. And so competition and ultimately, development, is weaker there.
But maybe I'm using a skew look. Perhaps the marathon is finally catching up with track, or perhaps it's just that it is no longer enough to win the marathon but people now want to see world records; and so it was the approach of the marathon that is catching up.
Either way, I still believe that the community with talent for 1:39.xx exists today but has not produced it because they've not applied that talent correctly.