I feed both felix and whiskas, but I do think they are junk food tbh, esp the dry food. If that's all your cat will eat, fair enough, but they do contain cereal (not all wet whiskas do - I think one of the supermeat varieties has no cereal) and that is not good for cats that are carnivores. They have been fed to cats for years because they are what are on the supermarket shelves and most people don't dwell much on what they feed their cats. And when you look at what is on the supermarket shelves for humans.... mostly processed junk! It can be hard to switch them over from foods like that to those that are pure meat, as their tummies have got used to one thing so dont always adapt well to something new, plus they are a bit like children who will refuse meat and veg and plead for macdonalds! They want the junk. Some cats will just have digestive issues generally and that complicates matters, but overall cats are designed to eat meat and not cereals. You are not a bad meowmy for feeding whiskas, but it isn't the best quality food on the market. It's a complete food and your cat may do fine on it.
The addition of cereal, soy and veg deriviatives is not directly related to price - sometimes I look in places like quality save and find foods that are cheap and are just meat, meat deriviatives and minerals. Meat deriviatives may not be ideal but I think in this country regulations do limit what goes into cat food and so there's nothing really bad in there. Butchers classic is a cheap basic food that is grain free and sold in most supermarkets. Hi life essentials is also under £3 for 12 pouches in quality save and is grain free.
I feed some dry and go for Applaws as it is grain free and Mosi loves it. I only really feed dry to help keep costs down. He has had runny poos in the past but they tend to be when I've given him foods containing grains. he does have dreamies which are junk food and contain cereal, but obv only small quantiites. the rest of the time he has hi life essentials (just meat, minerals and a bit of sugar) and he's fine.
The important thing is that if you want to change, do it gradually, to minimise the risk of upset tummies.
For most of us what we feed is a compromise. We have to bear in mind cost, availability and above all what they will actually eat! The best food in the world is no good if they won't eat it or it upsets their tummy. So we have to make a decision, be happy with it and don't let anyone make us feel bad if we aren't feeding the best quality food out there.