Looks like there are/were problems with TB3 and they decided to start from the speed of lowest TB.Maybe you missed that the rest of the new laptops were upgraded to Thunderbolt 2?
Airs first had 5Gb TB. Now this rMB has 5Gb usb3.1 "Gen 1". Next macbook probably has real 3.1 speed i.e.. 10Gb. That's the speed of original "full" TB1.
Then it gets interesting. Will they just put more 10Gb typeC ports or combine them with TB2/TB3 ports or dedicated dp1.3 ports?
Will Apple ever release another display to cope all these display options? Can ATD work just connected to mDP port of a computer?