> A friend of mine was proud, once, that his new car included an audio input jack in the dashboard. That's gone out of fashion now, but frankly it's still the ideal setup.
I haven't shopped for new cars recently and I hope this isn't true! I like to listen to audio books and don't really want my phone involved. While AUX jacks aren't the only way to do it I like the option.
A 2006 car I had lacked and AUX jack and had a CD player. After experimenting with FM transmitters (beyond horrible) I eventually had to install a new head unit myself. In the past I used to take it to a car stereo place but they had all shutdown by the time I needed them. While there were directions and adapters available it was a pain because I had to tear apart a lot of the dash trim to do it. I ended up wishing it came with a cassette deck because those adapters you mention are cheap and work quite well.
I still have a 2011 car with CD player and no aux jack which doesn't seem to support stereo replacement, or at least it is going to be an even bigger project. But since it can read data CDs full of mp3s I just burn audio books to a few CDs and the swapping is so minimal it isn't a concern.
My wife's car is a 2019 and it seems to have corrected all the wrongs here, it has an aux jack and CD player but it will also read USB sticks. It's actually nice to be able to switch to different systems. Of course I hardly drive it and she mostly listens to audio CDs still!