I'm confused. Wasn't the last car capable of a having an after-market head unit installed manufactured a solid 10 years ago? I fail to see the point. The number of such cars is on a rapid decline. For collectors if you're going to buy an ancient car, then wouldn't you be buying it for nostalgia's sake and want the old crappy radio that came with it?
Pretty much, unless you buy a car with just the basic radio. I traded in my 2003 Murano for a new car last summer and it was 10 years old. The A/C controls were built into the Bose radio system. It took an aftermarket company 7 years to release a control panel that would let you swap out the stereo system. Once they did, I was able to replace the crappy Bose with a Kenwood Navigation system without losing any functionality.
My new car is a 2014 Jeep Grand Cherokee and I have the Uconnect 8.4 infotainment system. It's a good system, but everything is built-in to the unit. It would be necessary to get an add-on module on top of a new stereo system to retain all of the current functionality. The only company that I know of that is on the cutting edge of developing these modules is iDataLink (http://maestro.idatalink.com/). They don't have a module for my car yet, but they do have modules for GM On Star, Ford Sync, and older Uconnect systems.
Personally, I wouldn't replace any stereo system to get CarPlay. In my opinion, its a dumbed down system because it requires you to use your cell phone for the majority of the functionality. Plus, if you have a family who all drive the car, everyone will need an iPhone. Give me a unit with built-in Garmin GPS any day....