All of the quests, game logic, AI (including brain dead NOC interactions), voice acting, etc, are exactly the same as in the original and are actually driven by original GameBryo engine. They only rerecorded some lines to add unique voices to NPCs of different races and made some minor gameplay tweaks. The only major changes are graphical - UE5 is used for rendering, all meshes, textures, landscapes and animations are redone. It’s more than a typical remaster like Last of Us, but not exactly a full remake.
I honestly did not expect Starfield to have actual flyable spaceships and vehicles. That was a pleasant surprise, so Bethesda evidently has not stagnated completely. The problem is Starfield has issues with many other game elements (like loading screens, mediocre worldbuilding, etc). Also the fact that it was simply a game in a different genre than previous Bethesda games didn’t help. People expected a handcrafted open world a la Fallout 4 but got a kind-of-procedurally generated sandbox.
They are probably an experienced team members that are needed on ME5, and management decided that they can’t distract them. And these new devs would spend a lot of time figuring out a completely unfamiliar codebase (also creating a proper remaster would also necessitate a lot of low-level changes to the engine which will make it much harder). So it’s a business decision, not technical one.
Remaster has some changes to leveling and combat, so mods that touch this will need to be updated for remaster.