For the average car sound-system, I doubt the quality improvement from the cassette->CD upgrade is going to be all that appreciable. The upgrade to CD only makes sense for a decent sound-system, or if cassettes are no longer available.
This seems comparable to the FM->DAB (or DAB+) upgrade. I don't know about the usage patterns here, but I suspect radio is more often used in vehicles than at home with nice speakers, or with headphones. Perhaps it's different from CD/cassette in that regard, or perhaps not.
CDs are also just as recordable as cassettes.
I suspect that CD players are probably beginning to disappear from cars, in favour of SD cards, USB & Bluetooth - a lot of the youth seem to be moving towards mobile phone-centric audio entertainment in cars as well as everywhere else.