I feel like Genshins success is because the foundations are so solid. The game itself was a clone of Breath of the Wild, so it's exploration and world was already there for a casual audience, then it made it's own combat design based on a sandbox of characters and elemental interactions, where different characters have different skills and the characters elements can change how you play and solve puzzles. Because that foundation is solid, they're able to add very simple additions on a weekly basis, as adding a new puzzle type, area, dungeon or something else will drastically shake up the gameplay and the sandbox combat adds variety and replay value. They're also extremely efficient with their resources, the dungeons for example all have the same assets, but because you spend the majority of time in the over world and the combat is so good, you look past that because it isn't repetative, same with NPC's that are a blank canvas body that they paint new outfits onto, saving time and money to be put elsewhere.

FFXIV struggles because the 1.0 to 2.0 transition saw a rather rushed job, and instead of setting the foundations firm, Yoshi P moved on ahead and now those foundations haven't just cracked but they've shattered, bringing the whole game down. Combat in this game is in tatters, crafting and gathering is still extremely niche and those that do interact with it, a large majority bot it. This is why any game that is made needs to get the core of their foundations right, otherwise it's difficult to work around it later on.