In short, the original game was very much "if it ain't broke, don't fix it" for the main combat loop. Just needed better questing, better writing/VA/cutscenes, more varied activities, more stuff to mount, swimming, more dinos, etc...to expand on the game.
Forbidden West added a bunch of that but instead of being an Uncharted 1->2 or Assassin's Creed 1->2 sequel that's just bigger and iterative, they basically changed the core combat and combat/crafting loop into its own unique thing. Forbidden West is a very different playing combat game than Zero Dawn. Some people like Cyber prefer the new combat, some people like me liked the ZD combat better. They're just very different in combat. I think a big part comes down to what kind of player you are. ZD rewarded slow aiming stealth gaming players, FW rewards fast action, fast aiming players.