DD1 went through 4 sizes of shoes last year, and that's an improvement on past years. DS however wore his out with 2 weeks to go last year and they still fit, so that's what he's starting school in. I generally only replace as needed too.
Saying that, I bought DD1 a new bag this year even though last years was fine, as last years was an old work bag of mine (obviously used to make them tougher lol), and she pleaded for 'her own'.