I am trying to figure out when the industry might need to transition to 3.4gbps data transfer rates.

In order to require this level of bandwidth, the system would need to do

a.) 1080p at 120Hz transmit rate + >= 8 bit color
b.) WUXGA at >= 10 bit color
c.) UXGA at >= 12 bit color
c.) 1080p at 60Hz transmit rate + >= 16 bit color

Which of these four is the most likely scenario and when?

Previously, it was said that there would never be transmission at 120Hz because it would be a waste of bandwidth. It seems that the industry evolution is for more and better and at some point this will happen.

The 1.3 standard was written with more colors and more pixels and more bandwidth in mind. Why wouldn't manufacturers try to take advantage of all this?

They will take advantage of it.
But in the usual slow way to promote upgrades every couple of years.
Also so that they can sell thru current (and upcoming) stock.
I think that you will see both A & C in the next 12/18 months (depending on the economy).