All antifreezes comprise of two components, antifreeze (usually glycol) to prevent it freezing and corrosion inhibitors. The antifreeze component lasts indefinitely (although it will be weakened by dilution if you continually make up losses with water. The corrosion inhibitors get used up as they combat corrosion.
In my experience a 50% mix of Ford 10 year antifreeze does what it says on the tin, but after 10 years the corrosion inhibitor will be all but used up.
The Morris Ankorsol simply replaces the lost corrosion inhibitors, but unlike the Ford corrosion inhibitors only lasts 2 years, so must be replaced at that interval if you want to prevent corrosion.
It is a lot easier to add Ankorsol (at a rate of 2% of system volume) than it is to drain and replace all of the antifreeze in the system, 45 litres in the case of my boat.