To point it simply you don’t need voice acting for fate stay night because it has a lot of stuff, explores characters, gets into the details of interactions, scenes and the feelings of characters. While it would struggle if you took out the visual part of the visual novel since it is written with the visuals presenting further context to the writing and changing the nature of what needs to be shown or said, you can sort of get by to a certain extent?
FGO doesn’t have that detail, and is a lot more limited visually. So you use voice acting to imply and flesh out characters and fill in the blanks.
F/SN can, and I would argue has, with the Realta Nua version, benefited from the voice acting. However it’s a supplement to something that’s already very solid and there. That I argue isn’t quite there in FGO, not just because of any debate on execution, but inherently through its medium and the way it’s structured as a mobile game.
so yeah, it wouldn’t fix its problems but it would do a lot to make what it does have go a lot further and be more palatable.