The service is called for at 5 year intervals because the coolant becomes acidic. This is because the coolant is in contact with various ferrous and non-ferrous metals in the cooling system.......primary examples being the iron block, aluminum head, aluminum radiator, steel water pump blades and so on.
To this, add some solids that separate out of the coolant itself over time.
I don't embrace formal flushing since flushing compounds are often introduced that may not be fully cleared at the end of the process. The compounds, over time, can actually induce rapid corrosion.
As a result, I lean towards the drain and refill process.......this may only refresh about 70% of the coolant but the anti-corrosives in the fresh coolant are strong enough to 'recharge' any remaining coolant.
But, by all means, yes, service the cooling system and replace the cap in conjunction....it has done its time.