the big thing with ethylene glycol coolants, whether it be the traditional green stuff or the new extended life/dexcool stuff (still ethylene glycol based) is that the E.G. won't break down inside the engine. As long as you maintain the proper 50% E.G. & water mix, you will still have the same low freezing and high boiling point. And you can change regular coolant after 10 years of service and it can come out very clean and very green, but there's a lot going on that you can't see. The whole problem is with the corrosion inhibitors and other additives in coolant to protect the various metals in an engine from corrosion, electrolysis!, and scaling/rust build up. You also have to worry about the additives falling out of the coolant and leaving deposits, and when the silicate falls out of the coolant it forms the green goo which affects coolant flow and heat transfer. Although it's true that the new extended life coolants today (i.e. dexcool) do last longer than conventional green coolant, the key word is "longer". They do deplete. So to say you "never" need to do a coolant flush or a coolant change, you are incorrect.
And the bad thing about coolant related problems is they usually are catastophic. Once in a while the radiator or heater core will clog and by just replacing them you can get by. But when a head gasket (metal) fails or a water jacket corrodes through or blocks up due to corrosion or electrolysis, or if you have cavitation in a diesel motor (where holes are pitted into the cylinder liner on the water jacket side and coolant enters the cylinder) you usually end up junking the motor or doing a complete rebuild. Cavitition is a big thing with older diesel engines and we have to routinely add coolant additives to maintain nitrite levels to protect cylinder liners from cavitation and the whole system from corrosion in general.
And the bad thing about coolant related problems is they usually are catastophic. Once in a while the radiator or heater core will clog and by just replacing them you can get by. But when a head gasket (metal) fails or a water jacket corrodes through or blocks up due to corrosion or electrolysis, or if you have cavitation in a diesel motor (where holes are pitted into the cylinder liner on the water jacket side and coolant enters the cylinder) you usually end up junking the motor or doing a complete rebuild. Cavitition is a big thing with older diesel engines and we have to routinely add coolant additives to maintain nitrite levels to protect cylinder liners from cavitation and the whole system from corrosion in general.