It's more the overall cost of all the work and other stuff with an older car - I start doing the "man maths" and thinking for not much more I could lease a new one....especially when you think that, unlike 911's, my old C Class is still depreciating, albeit slowly.
We had our first big bill - £1000 - on our C Class last month. But we've had it nearly 5 years and only paid £8000 for it (main dealer, approved used) so I think we're well ahead compared to leasing a new car. The trick I guess is to get rid of them before they have too much mileage and wear & tear becomes really significant. Ours is coming up for 90k miles now ... planning to keep it for a while yet.