My mom's kitty lived to the age of 18 on not much more than Friskies and Tender Vittles (we were on a tight budget back then).
My RB girl kitty ate only Science Diet dry food for her 15 years, and she did fine. The free-feeding dry and twice a day moist are what I'm giving my boy kitty, since he was started out that way. I always wondered if it was the best way to do it, so I'm glad to hear that it's common practice.

I'm very fortunate that my kitty has never had any digestive problems or hairballs, although he does have impacted anal glands fairly frequently. I've never seen him vomit in the 4 years I've known him. When my mom-in-law owned him, she was sprinkling Barley Cat powder on his moist food, so I kept that up too. I think that's what keeps his digestion working so well. I also made him a raised feeding platform, like they have for dogs, so he stands while he eats. Even though my cat is WAY skinny, he maintains his weight consistently from year to year, so the vet said he's just destined to be a skinny guy. I offer him healthy table scraps like chicken, fish, etc., but he's usually not interested (he's such a supermodel ).