Tons of headsets support mp3 over A2DP, but few sources do. It was intended to enable low-power operation by streaming the mp3 content directly from source to sink without transcoding. This seems perfectly sensible to somebody making a media player, but for smartphones it means you have to come up with something else to do with your UI tones and notifications and whatnot (because you can't mix them into the mp3 stream without decoding and re-encoding, defeating the purpose of mp3 passthrough).
For the same reason, ATRAC is also a defined A2DP codec: so your MiniDisc player could stream directly to an A2DP sink. I'm not sure if this was ever implemented by anyone.
I'm pretty sure all these codecs (except AptX) were part of the A2DP spec from the beginning. SBC was the first implemented because it's computationally trivial and royalty-free. I'm not sure it would have been practical to encode mp3 in real-time on a featurephone in 2004.
Anyway, the limiting factor of BT audio quality is the codec, not the radio. AptX is ~384Kbps for 16-bit stereo, and BT4.0 has a raw capacity on the order of 25Mpbs. Further increasing the interface speed isn't going to change the audio quality