My stereo has neither USB nor bluetooth, and damned if I'm gonna buy a new stereo with my new phone.
A bluetooth receiver costs all of $5... I've got a couple to retrofit otherwise decent (and expensive) older car stereos/entertainment systems. The sound quality of bluetooth in general is no match for hard-wired, but it's an option.
My concerns are much more practical... FM radio is a nice option to have in phones, and the headphone cord doubles as the antenna. Bluetooth obviously can't do that.
I also like the no-brainer ability to just plug in a cord and everything works... No navigating menus to select the device you want to send sound to.
I also flatly refuse to hassle with a bunch of different devices with separate batteries that need routine recharging. That's the only reason I don't carry around a bluetooth earpiece and keyboard with my phone... If they could both securely clip-on to my phone and have contacts allowing them to recharge their own batteries from my (larger) phone while not in-use, I'd love to have them. Until that happens, no go. My corded ear-buds (sitting in my bag for years) will be ready to go whenever I want with no maintenance.