I really don't want to sound rude, but let me try and explain this differently.
Let's say Gamebryo is the foundation you're building your house on. Your house is named Creation Engine.
Gamebryo came out in 1997 and went through constant changes as any game engine will. Nevertheless, its codebase is from 1997 and has not fundamentally changed, it's just been iterated upon.
The Creation Engine was built on that foundation and reworked to suit their evolving needs. This engine was released in 2011, and since then every single feature that has been added to it has been added to a codebase from 2011, which was already an evolution of a codebase from 1997. CE2 is just CE with a few things added on top.
Now compare it with Unreal Engine. While UE 1 through 3 all share the same baseline, and it's semi-easy to port between them, UE was developed over 11 years and is pretty much a new engine, with a new foundation. It's pretty much impossible to port your game from 3 to 4, and instead, you have to almost re-write your game from scratch. UE5 is an evolution of the foundation UE4 build
Some context:
http://xingthegame.blogspot.com/2014/04/we-made-switch-to-unreal-engine-4.html
Your knowledge of Software design is, quite frankly, lacking. It's not the Creation Engine that makes their games special, it's actually their Radiant AI and Radiant Story technologies, as well as their Mod support. Anybody who understands a tiny bit of game/software development can tell you that said technologies and mod support can be rewritten from scratch and perfected, as well as integrated into the engine.
Both of these technologies were originally written for Gamebryo.
Exactly. Imagine Starfield with a modern, tailor-made engine, created with modern technologies in mind, free of legacy bugs (pretty much every single bug we've seen has been seen in bethesda games in the past, one way or another)
And Fiat made good cares with FIRE engines. Doesn't mean those engines are good.
Again, mod support has no bearing here since ANY game engine can be created to support mods.