I do, also agree with the "dont fix what's not broken" - But, however with some of these poorer quality foods, they might be doing well on it now, but it could be affecting them long term, too. After eating lower grade food for several years it could start to catch up with them, then at that point the problems could be irreversable and you could possibly have to deal with hefty vet bills later on down the road. Some dogs do very well on lower grade foods their entire life and live healthy and happy, but many do not at the same time, and a lot of people tend to blame their health problems on genetics or old age, etc. and yes, some could possibly be because of this, but in my opinion, most of it is their diet.
I do not agree with anything vets recommend for diet, and that's just a personal thing. I'm a raw feeder and many vets don't agree with this, but I strongly believe its the best diet for MY dogs, and me, too. I have fed higher quality, grain-free kibbles, and I do still keep a few small bags around when I need to feed it for whatever reason if i'm running low on raw, or camping outdoors and I prefer not to attract other predators to camp with the raw food, and my dogs have done reasonably well on higher quality, grain free kibble (I also don't beleive in feeding grain inclusive or anything with corn, wheat, soy, animal by-products..etc), BUT even when the higher grade kibbles are eliminated from my dogs' diet, I notice huge differences and they do a hundred times better with no kibble what so ever in the diet. This has led me to be totally biased and now I have a hard time feeding any kind of kibble, but you do have to do whats right for the dog in the end. There is BETTER stuff for similar price as Purina, without all the nasty ingredients. I would encourage to look into and do some research of other brands, maybe with some similar ingredients that you know your dog does well on - and go from there, as a 'preventative' measure. I'm not sure and dont' quote me on this, but wasn't it Purina that was using some very questionable by product ingredients, some of which tested positive for the chemical used to euthanize pets? I dont know.. one of those big box brands..anyways, maybe its just me but I don't think I would feed anything from a company that put euthanized pets in their food. Maybe that was just a bad rumor... but it wouldn't surprise me, unfortunately.