>I ended up buying a $80 bluetooth adapter for my wired headphones

What's the point of a headphone if you're listening to MP3/AAC → A2DP transcoded music?

it turns out that even mp3 encoding audio doesn't actually stop it being audible to other people nearby when played through speakers, the headphones on the other hand, mean only the wearer can hear it...

True, but I was talking about a headphone being pointless for A2DP because the transcoding would degrade audio quality to the point that even a cheap earphone would be sufficient.

Or if someone wants the headphone form factor, then 80 bucks is more than enough for an ok pair of Bluetooth headphones that won’t sound worse than the A2DP transcoded stream already is.

FYI, I think your terminology is slightly non-standard though - (UK) english normally would describe any device worn on the head which allows you to listen to audio as either 'headphones' or 'earphones' interchangeably, with the distinction you refer to being indicated by using the modifier 'in-ear' beforehand...

Also the cost differential that you seem to suggest exists between them is not apparent - I have seen examples of both form factors of bluetooth headphones as cheap as GBP 10.00 (or less) at a minimum, and you could spend way over GBP 500.00 on either kind if you so desired - you get what you pay for, though...!

My three priorities are convenience, comfort, and noise-cancelling (call me a heretic, but for me A2DP is "good enough" quality-wise, so I don't care about that).

I already have a pair of super-comfortable noise-cancelling headphones that I carry everywhere, and getting an adapter is more convenient than carrying wired headphones in one pocket and wireless headphones in another pocket :P