I'd say that a good 70% of the cars on the road are neglected, either in terms of service, (or rather the lack of it), or how they are driven, (ie, thrashed from cold).
When I bought my Omega the previous owner had three different size tyres on it, the coolant was like water and the oil like tar. Pretty much everything has needed sorting.
Years back I had a 1.7 Ford Puma. Because of the nikosil bore liners these needed regular oil changes and a close eye keeping on the oil level.
I opened up the bonnet and couldn't immediently see the dipstick. I asked the seller where the dipstick was, but he didn't know. In two years and 30'000 miles he had never had it serviced or even checked the oil. And this was on a three year old car! Needless to say, when I did find the dipstick it was bone dry.
I walked away from that one.