I'm not disagreeing with you on the point you're getting at, but no game engine from a studio that manages a franchise is truly "brand-new". It simply isn't feasible.As well
- Brand new Engine, all Civ 5 code was recycled from Civ 4, you could find remains in the files.
- Increase the limitations of the engine. I mean, I think to this day the graphical engine for Civ 5 is rather quirky, with grey tiles and several other aspects still slowing down the game.
You'll probably find CiV code in the Civ VI engine, nomatter how well-built it ends up being, or how fantastically it performs. I remember reading an article pointing out that Civ 1 code was still knocking around the codebase.
Regardless of how it was made though, here's hoping for something that's definitely a new version compared to CiV.