Since I cannot afford anything with a Mark Levinson audio system, the vast amount of cars I can afford have absolute shite for radios. I would much rather something aftermarket, thats completely tunable with a small 10" sub in a box in the back of the car somewhere. Sad really that people do not care about music in their cars anymore.
Here is the thing, even cheaper cars that are less than 5 years old have decent radios. I'm thinking here cars like Honda Civic, Elantra, etc. Certainly they have all the inputs you would require such as Bluetooth, AUX jacks, USB inputs, etc. Fifteen years ago, you could only get that stuff by installing an aftermarket deck. That was the primary reason I installed a deck in my 2003 Civic, way back when. I wanted to attach my iPod to it. When I bought my 2008 Civic SI, and it had an AUX jack on it, I didn't feel the need to upgrade the sound system as I could play my digital library. The same went for my Altima, and now my Grand Caravan has a built in DVD Player, Hard Drive, Aux Jack, USB Input, and Bluetooth. So many ways to get music from my phone to the car (I usually use Bluetooth to stream Spotify these days).
So, from an input perspective, even going back almost 10 years there is no reason to switch out decks.
So what about sound quality? Well, that's a different story. Surely, the sound system in my van is shite. I agree completely with that. But the thing is, good sound would be wasted. The van has plenty of wind and road noise, lots of vibrations, and the sliding doors creak every time we go over bumps. Putting in a whopping good sound system with the aim of excellent sound quality would be completely lost on this van. The same goes (probably to a lesser extent) for most other cheaper cars out there. They aren't quiet enough to appreciate a decent sound system, let alone audiophile grade. And the cars that are quiet enough to be able to appreciate good sound, i.e. all the German cars, Lexus, Infiniti, etc, all usually come with a good sound system from the factory. I.e. Lexus and Mark Levinson. Or VW and Fender, or whatever.
And then we circle around to the integration. Most vehicles in the last 3 years have integrated their head units beyond belief. Everything from controlling climate control, heated seats, to steering wheel buttons and rearview cameras. If I were to replace the deck in my Grand Caravan with an aftermarket one, I would lose all of my steering wheel buttons (which I find indispensable), and possibly my rear view camera. The latest U-Connect head units control heated seats and tons of other stuff. My mom's Land Rover can access the entire vehicle setup menus in the head-unit. She can do everything from check tire pressures to vehicle settings, everything. The integration is such that if you replace the head-unit, half the features of your car are rendered inoperable and useless.
Fair enough, this problem of integration is a fairly recent thing. If you buy a 10 year old or even 5 year old used car, the head unit probably isn't fully integrated yet. But then you circle around to the NEED to upgrade the head unit. It just isn't worth it, as you don't gain any extra usability. The only vehicles that I could imagine somebody NEEDING to upgrade the head-unit on, would be a vehicle that is older than 10 years old, and then it's to gain accessibility above and beyond CD's, such as Bluetooth, USB input, etc.
Then we have the young people and their booming stereos. I suppose, that considering most young people buying a first car that is 10 years old or older could see some value in self expression by installing a head unit and amps, etc. And they would gain some usability from that by having increased input options, etc. But there will come a time when the youth of tomorrow are buying their first cars, and those first cars will be 10 year old 2016 Civics and Elantras, which already have pretty good sound systems, and are fully integrated. And those youth will have zero interest in ripping out a head unit, as it will provide zero benefit to do so. And then the aftermarket car audio industry will have died.