If the pink clear coolant has turned into 'muddy red crap' it is doubtful the car will last another 10 months, let alone another 10 years! Unscrupulous behaviour would be to replace it.....and then sell the car.

No, it does't disappear, nor does the anti-freeze disappear - but the anti-corrosive measures drop at end of rated life.

The 'anti-corrosive measure' is an organic acid (carboxylic) which reacts with metal surfaces to form a protective layer which lasts for ages. If significant corrosion had occurred, my 18yr old coolant would no longer look like raspberryade.

Even without oxygen, many cars will corrode through the use of different materials.

I presume you are thinking of e.g. an alloy head in proximity to an iron block. I think this would be negligible. I have never found it to be a problem, even in our old Passat which did nearly double the mileage of our current Focus (135,000). Indeed, I think that replacing coolant regularly, thus introducing a fresh supply of corrosive suspended oxygen, plus the associated risk of breaking something or creating potentially damaging air locks, probably does more harm than good. Actually, it was a problem once; removing a 22yr old alloy water pump cover from my TR7's iron block without breaking it took me two days of tapping, blow-torching, freezing and prising!