A 7+ year old PC is fast enough for most. This is the only reason PC sales are not what they once where.
This pattern seems to be settling in with phones and tablets now, too. Sales are flat, but people are using phones/tablets more than ever. For most people, it makes no sense to buy a new tablet or phone every 1-2 years. My wife's iPad Mini 2 continues to perfectly serve all her computer-related needs.