Word is that after six months, opened, there is a serious loss in vitamins. So many people do replace food.
I consider it relative as a loss. I use food for up to a year, and see absolutely no bad effects. There are probably a thousand fish that die from obesity (a real killer in over nourished fish) for every fish that is short changed by slightly older food.
You shouldn't use just flake or pellets anyway, if nutrition is a concern. Processed food is processed food, for us or for them. Depending on the species, there can be a lot of better foods you can work in and out of the diet for variety.
I've used fish food that expired years ago lol and never had any problem with it, I always seem to end up with multiple containers of fish food (buying second hand tanks, side of the road finds etc, etc). The oldest was in a container that didn't look anything like fish food containers look today (maybe from the 70's or something??), it was actually one of the best fish foods I've had and the fish were crazy about it lol. It was in a red and yellow container, I wish they still sold it.