In working with sick animals, I always recommend a specific diet according to their animal friend’s condition. And within that specified diet, there are always at least two protein choices and usually more.
Besides encouraging the client to stick to my prescribed diet, I also encourage them to vary the proteins and vegetables versus picking one of the choices and that’s the meal over and over. Each protein and vegetable choice has something slightly different to offer PLUS variety is just nice. No one wants to eat the same thing for months and months without other choices.
I think variety is especially helpful for finicky cats. They rely heavily on smell, and I believe their nose becomes immune to foods that they get repeatedly; hence, they may start shying away from their food. As humans we become immune to smells that we encounter repeatedly, therefore, it makes sense that a cat may start showing less interest in its food because it can no longer pick up the aroma which they rely on to feel confident to bite into.
Variety does a body good, and it makes meal time more enjoyable!