I just sounds like a pain in the butt to have to keep switching their stuff out. They've always been happy with using the same one every day. They get a new one each year. I've never noticed any kid not using the same backpack everyday
If the zipper breaks or it rips, sure. Otherwise you stick with what you get. DS chooses his backpack when it needs replacing. If he changes his mind he's SOL. He chose it, he wanted it and that's what he got.