Tyres - how long should they last?

My Audi A6 Avant has almost 9000 miles on the clock and I bought it from new. It had a slow puncture caused by a nail in the front driver's side tyre. The guy at the tyre specialist said that the tyre had only 3mm of tread left, and I should consider replacing both front tyres. The back have 6mm tread. Do Audi supply the cars with small tread, as surely 9000 of mostly motorway miles by a sedate driver would not wear out the front tyres?
A tracking problem or disparity in pressures has cause dragging and the excess wear. A6s have overhanging longitudinal engines so this is critical. Once the tyres are replaced (not before) the car needs a four way laser alignment at a branch of www.alignmycar.co.uk/
Answered by Honest John on

Ask Honest John