If Apple ditches the 3.5mm jack, they will include wired lightning earbuds in the box, not bluetooth. I do not particularly see bluetooth use accelerating for headphones. It seems to me from my travels that it has remained relatively static. Most people still use wired headphones. If anything, I've seen a decrease in wireless headphones since bluetooth has become so ubiquitous in the car.
You don't have to take my word for it. Here are the top selling headphones currently on Amazon:
http://www.amazon.com/Best-Sellers-Electronics-Audio-Headphones/zgbs/electronics/172541
7 of the top 20 are currently bluetooth. Yes, that still means the majority being sold are not... but that puts us at just the same point where many other technologies Apple has dropped were: many people were switching but most were still using the old technology.
Oh - you mentioned a while back that even bluetooth headphones still have "wires". That is just now starting to NOT be true (and yet another reason why Bluetooth earbuds are going to explode in popularity in the very near future). Check out any of these:
https://www.getkanoa.com
http://www.engadget.com/2016/01/04/altec-lansing-true-wireless-headphones/
http://ownphones.com
https://www.truebuds.io
(and more)
Yes, they are pretty expensive (and some are ugly/clunky) but these are just the beginning. More and more will pour into the market over the next few years.
Many of these also eliminate some of the other concerns with Bluetooth. For instance, many come with a charging case that can charge the earbuds wirelessly just by placing them in the case when you're not using them.
There is a lot of innovation happening in Bluetooth headphones/earbuds right now... and I fully expect to see apple nix the 3.5mm port in favor of it very soon.
As an Amazon Associate, MacRumors earns a commission from qualifying purchases made through links in this post.