Cars just aren't built to look pristine after 10 years these days - maybe an exception to that would be Merc S-class/Bentleys etc. My disc hubs had decorative rust on them from almost day 1. Brake disc shields will rot, springs and struts will collect surface corrosion at the very least - your actual car body should remain pretty resilient though. Most cars with good residuals take a real dive in retained value when they hit 5 years old and the dealership network is no longer interested in it for used stock. At that point when they have to compete with other cars, you will not see a 5/6 year old example of an Audi S3 commanding 3x the price of a well maintained Focus ST of a similar age - something has to give.
When you consider how much it is to maintain an old car and how much your 10 year old S3 will have lost in those 10 years, there isn't much in it vs annual cost of running a new one (less than a grand a year in it). Buy a new S3 at decent discount (£30k paid for a £33k RRP model) and it's probably going to be worth £20k after 3 years, it's cost you £3333 a year in depreciation and will have needed to do nothing to it except replace the front tyres and give it 2 or 3 services. Buy a new one for £30k, keep it and have it worth probably £4k at 10 years old, you've lost £2600 a year, will have replaced the brake pads twice and maybe the discs once, maybe a new clutch, probably shocks all round once, maybe a timing belt and a gearbox oil change - it all adds up. 7 years without the reassurance of warranty. There's next to nowt in it. Consider any new innovations that might become standard on newer cars in the 7 years following year 3....
On a new car with exceptional residuals, it usually makes sense to change every 3 years, VAG engineer the financials of ownership to make it so.