Thats why I ended up buying a new bike. Bought a used mountain bike and was using it for commuting and riding trails with the kids. Started wanting to put on a new seat, smoother tires, etc. By the time I changed it into a hybrid I would have had more invensted than a new bike but it would still just be a $150 used bike to insurance if it were ever stolen. So I just bought a new bike for the family rides and use the mountain bike as my commuter so if the already worn out seat were to get messed up its no big deal.
Of course now I want to upgrade components on both.