What you need to remember about water wetter is that although it does transfer heat more effiecently from the radiator, it also absorbs more heat from the engine, so many times it doesn't actually run cooler, but it is dispersing more heat. Also Glycol tends to have more consitent temperatures since it actually holds more heat and has slower heat transfer.
It's possible that it slowly becomes contaminated over time. Personally I ride onm and off throught the winter and I don't have a heated garage, so around early November I will be changing backl to glycol, then back again in the Spring.
The active ingredient is basically soap, it breaks the surface tension. The heat transfer characteristics shouldn't really break down. The other additives will break down from chemical contaminates due to combustion. 1.5 years is longer than I would recommend. The use of distilled h2o will also help prevent some corrsion.