You're oversimplifying - releasing a new sequel doesn't mean you're iterating on the same engine as the previous one. In fact, I've never worked at a studio where this wasn't the case - even if we weren't making engine changes we still updated the code model enough to warrant re-writing a good portion of the game.

It's a tragedy of the modern day - most people have no idea what's entailed in making a game. No idea at all. So you assume it's like any other product, or it's anywhere near as simple as it seems. The reality is code is very complex, game code is very intertwined, intertwined code is very fragile. It SEEMS simple, but your opinion is irrelevant until you see how many different ways the "simple" thing you're changing is referenced across your engine.

I was working on an AAA game once - we fixed a bug that updated the way character models interacted with physics meshes, then half of our textures broke within what you'd assume was a texture applicator in a vacuum. Nope! For ease, the mesh generator had two references within the texture applicator and we shit on everything with what was otherwise a 4-line code change.

What I'm telling you is, you're making way too many assumptions. In fact, so is the video. Why is anyone confused how the concept art could go from so ornate to so simplistic in the final product? Dudes. Stuff gets cut all the time. I'm sure most of the UI guys over there are really great. For some reason or another, apparently, they ran out of time or resources or whatever. You think they actively chose to go from that beta concept to what we see now? Because they did a "crap job"? Like they're literally just sitting there all day trying to figure out how to do it, but they simply cannot.

It's amazing the general public thinks they have any idea how games are made.