(What happens to coolant properties that are regularly run in a 17 year old BMW "sealed" radiator?

I never open the radiator cap, or the reservoir tank cap. ( I have never observed a radiator or hose leak; I have never added coolant, and I have never observed a coolant loss in the

transparent reservoir tank. I am using a 14 psi reservoir cap. Where does one dispose of drained (hazardous) coolant if it were to be drained and flushed every couple of years?(What happens to coolant properties that are regularly run in a 17 year old BMW "sealed" radiator?They disappear and cooling system corrodes into oblivion.