To quote an old song: "Both sides now"
Right off, let me say that I had been feeding Purina O.N.E. and my cats were doing fine on it - no medical problems. However, reading forums, it seemed to me that grain-free was the way to go; however, on a very limited income (SS and investments - no pension), switching to a higher-end food seemed impossible. I did, however, find Taste of the Wild to be approximately the same price as the Purina, so I switched to it a couple of years ago. I've since found that buying it in the 15-pound bag makes it cheaper than Purina from the grocery store. What I've noticed is that 16-pound, almost-10-year-old Mordred seems to me to be more active than previously. I can't imagine that being caused by his aging! So I guess it was the change to grain-free. He didn't lose any weight on it, though.
Wanting to add some canned food at least occasionally, I was again stumped: I simply can't afford the high-end cans when feeding three. However, on another forum, someone mentioned Dave's Pet Food. I checked it out: good ingredients, no gluten, no grain - and surprisingly affordable. My three now get some of Dave's canned each evening.
On the other hand, years ago, before premium food was ever dreamt of, I fed grocery store food because that's all there was. Cassandra lived to almost 17, and Kalli made it to almost 20 on that food. Kalli would get seriously sick every few years, although the most serious - myocarditis - had nothing to do with food. Cassandra was never ill; I had her sent to the Bridge when she threw a clot. So, feeding grocery store food will not kill your cat; as long as it has the AAFCO seal, it has all the proper nutrients - although the sources of those nutrients may not be the best or most efficient.
I do have one "pet peeve" that I didn't see here but will throw it out there anyway: some people simply do not have the finances to buy the premium foods; they do the best they can and feed grocery store food because *that* is the best they can do. If they come to a forum where that food is called "junk," "crap," or "garbage," - as I've seen on another forum - they're made to feel inadequate, unworthy pet parents. I know that's how I felt when I was feeding Purina; to call the food I was feeding by derogatory names is to berate *me*, not just the food. The compassion we extend to our furchildren should teach us to extend it to others who may be less financially secure. ;)