"I always replace the battery at four and a half years (with a Halfords one). I reckon it is not worth the risk to try squeezing any more out of one. But that's me."

Quite. People must do as they see fit. However, I can't see the point of unnecessary expenditure. It's clear from this thread that many batteries last for a long time. Usually you get plenty of warning before they fail completely.

My Mazda is a 14 plate and still has the original battery. It lives outside. Every winter for the last few years I've expected it not to start on some frosty morning, but it keeps going. I'll replace it as soon as it fails - either to start the car, or the stop-start fails - a sign the battery is not holding its charge. If the car suddenly fails to start I'll suck up the consequences - cadging a lift, taking the bus, paying for a taxi or whatever. I will still have saved quite a bit of money.