Silicates do have a life, and it's a short one - two years. If left longer, build up inside the radiator begins, and the silicates actually contribute to cooling system problems.
Silicate drop out occurs after the coolant has been "spent", and the inhibitors drop out of solution. Silicates are blamed for deposit build up in radiators and heaters, causing engine overheating and poor heater performance. Once the silicates drop out of solution, dissimilar metal corrosion is no longer suppressed, and metal components begin to deteriorate.
Silicates are deemed to be abrasive in a cooling system, and are blamed for premature water pump seal failure.