Apple has done something here where their product is usually as good as it's going to get at inception. We're not going to see anything revolutionary because they simply can't do it. What you'll see in 12-18 months instead is a variant of the product.
For example, for those of you thinking that a new model next year is crazy if they release wireless charging in a month or two -- it's not that crazy when you consider that next years model might be totally different and might be sold alongside the current model - not replace it. For example, it could be "Airpods S" that fit snugly in your ear to provide noise cancellation, etc. We have no idea what that model represents, everyone just assumes that it would be an iterative replacement to the current AirPods. Not necessarily so.
As it's been stated, maybe 5% of all iPhone owners have airpods. There's a market here, and everyone who doesn't want to get AirPods has a reason, they just need to answer those reasons. It's not a huge technological problem. There are several Beats headphones for exactly the same reasons.
Personally I also think that the wireless version of the current product coming out in a few months will also have color options, and that'll totally revitalize the sales. Apple knows, vanity means a lot.