Fancy Feast isn't exactly a premium food, no. But the people who don't like it are generally the ones feeding premium, no-grain, organic, such like that, or feeding homemade. So relative to those types of foods, Fancy Feast is crap.
However, relative to dry food, Fancy Feast is great! Despite the price, Royal Canin isn't that great a food. That's all part of the marketing hype of the food manufacturing business. Price does not necessarily equate with quality -- probably the best way to put it is that the truly good foods aren't cheap, but being expensive doesn't mean it's good.
One thing I hate about it is that it contains corn, which is an ingredient that ought to be kept out of cat food. Even Friskies canned doesn't have corn. Corn is first of all a high-glycemic carbohydrate. It puts a lot of stress on their pancreas, which was made to process things slowly, not sugar rushes like carbs. (In other words, cats were meant to eat animals.) Corn is also hard to digest. While they
can digest it, you're giving them something that's hard to digest
every day. Your body would not like it if you ate corn every day either.
Dry foods in general have two issues. One is that that are nearly all high carb garbage. Look at the ingredients. Look at your cat. Tell me if they seem compatible. Remember a cat would eat a mouse or a rat or a bird if they can catch it or a rabbit. They would not eat a bag of over-cooked chicken meal and corn. I'm very happy your cats don't seem to like it as well, otherwise they'd probably be big fat pigs cause all those carbs turn to fat.
The other issue is that they are DRY. That can cause or contribute to problems with the urinary tract. They need moisture to help things flow through, otherwise junk can get trapped in there. I think it's less related to levels of minerals than it is to lack of moisture. They don't naturally drink a lot of water and are meant to have moisture in their prey with their meal. They generally don't drink enough on dry food to make up for the lack of moisture. Some of us believe this also helps contribute to kidney disease later in life.
Having had a cat with diabetes (from carbs) and kidney disease (probably part from dry food and partly from the stress diabetes put on them), I became a food freak. As far as I'm concerned, I gave my cat diabetes. I don't think I've ever seen a diabetic cat that wasn't on dry food. They can go into remission by switching to wet food. (Gotta be a reason for that, huh?)
So Fancy Feast is a much better food than dry Royal Canin.
And no, dry food doesn't clean their teeth. Old wives tale. Actually, the concept works, but in reality it doesn't. I'm not saying canned is going to clean their teeth either. Just that dry doesn't and it's not an excuse to feed it.
Granted, Fancy Feast isn't the greatest thing on earth, but I think it's a better choice than Royal Canin. Personally, I think the Royal Canin is like what's eating McDonalds.
So not only is the half can each reasonable if you insist on sticking with dry (it'll help but it's not much), but it's more reasonable to just dump the dry altogether. Yes, a better canned would be better, but the cats decide what they'll eat. Mine eat some Fancy Feast, Friskies, Nutro, Pro Plan... I'd rather they had more of some premium stuff, but they won't eat much, maybe a little Wellness or something here and there. I'm still happier with that than the dry.
Oh, and you can supplement with human meat, like some cooked chicken. Good quality protein. Just shouldn't be more than about 15% of their diet cause it's missing some essential stuff.
If your vet simply means there are better canned foods,
there are. But many vets have never even heard of them, and if he means Science Diet he's full of you know what. If he means not to give it at all and stick with that dry, then he's also full of it. Vets, by the way, are not nutritionists. There are exceptions, but the vast majority of them know very little about food. So much of what is out there is nothing but manufacturer marketing, and that includes the hype going out to the vets.
Do some reading:
Feeding Your Cat Know the Basi
Cat Nutrition.Org