Tell me, when you're doing your hobby, say, gardening, what would you do if some random schmuck came up to you and said "I really like peas, and you aren't planting any, so you suck. You should plant peas."?
All the time!
I don't think the line in jack is going to go extinct anytime soon. My current car, and a previous one, both had line-ins, and it was a major factor in the purchase of those vehicles. For the longest time, it was the only way to hook up an iPod or CD player (if the car didn't have one) or other things; I believe the satellite radios did it for awhile too, right?
Nowadays you can get a new car with an option to be "iPod compatible", but even if it works, it only works with iPods. Also, its an expensive option. A male-to-male line-in cable costs as much as $15 if you can't find a cheap one, but thats the upper limit. The options on new vehicles can add hundreds or thousands. Plus, the line-in "just works".