it's the corrosion inhibitors that give problems. Modern OAT coolants good for at least 5 years and probably 10.

Probably the life of the car. The inhibitor doesn't 'go off' and only 'gives problems' if it is neutralised by oxygen in the coolant. Modern cooling systems are sealed so that once the oxygen molecules suspended by the coolant water have been used up, no more can get in. I have no intention of replacing the oxygen-free 16yr old pink coolant in our Focus. The inhibitor has only ever been additionally taxed by a mere 200mls of rainwater added in the 13yrs of our ownership. Corrosion cannot occur without oxygen.

Rust needs oxygen - but other forms of corrsion don't, particularly engines with iron blocks and alloy heads.

Nothing to do with oxygen - the glycol breaks down with heat and forms acid species which attack various components. The corrosion inhibitors buffer the coolant to stop the attack. If it's still clear then no reason to change it.