They said in one of the livestreams that they originally had multiple ideas for future era units, but decided to put all the effort into the GDR instead.
And
this is, what puzzles me most.
The Civ6 devs did
almost everything to distinguish the new game from Civ5. They did choose new civs and leaders, even if the old ones would have been fine. They abandoned game mechanics and replaced them with something way simpler (and, imo, more boring. I refer to Civ5's strategic ressource system; the changes in GS are one of the main reasons why I am looking forward to this expansion!) just to do it in a different way. They altered the visual style. They changed combat calculations and how building bonuses work in cities. They changed how builders work. They altered specialists in buildings. They ...
the list goes on and on.
I am not even saying, the changes are bad (exept for the ressource thing and specialists); this is oftentimes a matter of personal preferences.
But they DID change a zillion small and large game aspects and oftentimes merely for the sake of change (or so it seems).
But when it comes to this GDR ... a unit I remember was
actually controversial in Civ5 and many would have been glad to never see it again in Civ6 ...
Can it
really be true that they had all those concepts (visual representation and game mechanics) almost finished and this person in charge (... sorry!!! I don't have your name at hands

...) wakes up one morning and has the revolutionary idea: "Hey, why don't we just add a Giant Death Robot? Man, THIS is a cool and unique idea!", walks into the bureau and stops all the work?
Again: I am personally
not opposed to the GDR itself!
But how does its addition compute with the desire to make as much as possible different from Civ5?
With all the game elements they could have re-introduced in Civ6, why did Firaxis choose
exactly the one unit (ok, the one alongside XCom) that was loathed by many in Civ5?