I would argue that in-car infotainment systems should be nothing but a dumb terminal into your phone. We already have devices with voice recognition, navigation, multimedia, etc. Why don't we have a way to just use that?

I've been looking for a single DIN car audio device that is nothing more than an amplifier with Bluetooth, but such a thing does not exist. My phone already does Music/Radio/Maps - all I need is the car head unit to connect to it and playback the audio through the car speakers. I've even started thinking about some sort of home built version using parts from a cheap Bluetooth speaker system (minus the actual speakers).

BlueBus is nice; but it works with the stock BMW head unit (nicely supports steering wheel controls tho). It was for pre-bt-enabled BMW's but I think it's able to be adapted to all k-bus-based cars, so roughly 1998-2010?

There's plenty of old-school single DIN devices that have bluetooth in them. You'll usually find they have a radio and other junk (e.g. mp3 over usb) but most are fairly simple and it shouldn't be hard to find one that stays in bluetooth mode all the time.

Yes, but they all look awful! XD

I completely agree. I wouldn't be surprised if this is what happens. It's partly there already.

Android Auto is pretty much that.